Let's consider purely technical aspects of icon design
1. Lines, slopes
One of the vital elements of the image is the outline of the object, i.e. rough border, which limits the object from the rest of the image. When designing normal-sized graphics, the artist occasionally cares about highlighting of objects with additional outlines. This is unnecessary because of the scale: even non-contrasted objects do not blend into a one. Pixel graphics is other story. Two solutions are available: either object and background colors have to be from opposite sides of the color chart, or the object should be divided from the background by contrast outlines and shadows. I will dwell on lines in more detail.
If we think again to the large scale graphics, we see that in order to define edges, we can use any (including the most complicated) angles, Bezier splines and borders. Anyway, the line will appear perfect thanks to anti-aliasing. When decreasing the image scale to the small icon size, situation changes dramatically. When each particular pixel is equally important and can ruin the whole look of the icon, anti-aliasing is simply not applicable. It comes that you should consider the possible line angles.
The angle you choose for the line, determines the step of this line. Because each line consists of basic elements, the combination of which defines its neatness and visual appeal.
For instance, a 18-degree slope contains many tiny 3-pixel lines combined with a 1-pixel downshift:
This is how nearly each line looks like. Basic element with joining followed by another primary element. Unfortunately, not each angle creates lines that look look neet and not annoying.
For example, look at 25 and 20-degree slopes:
It can be easily seen which line is more attractive. The 25-degree slope produses a line consisting of even 2-pixel primary elements. The 20-degree slope has a basic element containing of three parts: 3-pixel, 3-pixel and 2-pixel. The slope is more complex, therefore, when not supported by additional visual effects, it brings a feeling of untidiness: the primary elements have contrast which is too strong to hide from the human sight.
Now we can see the first rule. The most "correct" slopes must make as plain primary elements as they can. therefore, the perfect angles are 0 and 90 degrees. In those, 1-pixel primary elements are combined without downshift and form smooth lines. Less ideal are angles which form basic elements of 1, 2, 3 pixel with 1-pixel downshift. Of course, the lines will not be even, but the human brain will process the image and provide to the viewer what you meant to show to him. Also, the mentioned angles can be considered acceptable (but at a stretch): slopes with primary elements consisting of different lines shifted by more than 1 pixel.
It is the time to touch another problem. In the previous paragraph, I intentionally defined angles with basic elements of 1, 2 and 3 pixels as acceptable. Such tiny lines, especially when a great number of them is joined, look like a single whole. But what do we see if we change the basic elements to 10 or 20 pixels?
Here is an example:
It can be clearly noticed that the line loses its integrity. It is not a single line now; it is a combination of several lines, located near each other. Rarely any designer wants its composition to look that way. So, we have the second rule: if you use small angles, which make long primary elements, it must be reasoned and employed with great attention.
And, finally, the last aspect I have to mention about lines. I already wrote that the basic element has to be as plain as it can.
For example, a 25-degree slope can be drawn in the two (of course more of them exists) following ways:
In the first picture the primary element is a 2-pixel line. In the second picture, it consists of three elements: 3-pixel, 2-pixel and 1-pixel. The same angle but the image is different. The complicated basic element produced the line untidy.
This examples can be made for almost any slope, so in your works, try to simplify everything.
It can be said without an exaggeration that the color is the main element of your work; improper or poorly matched colors can kill even the best idea. What can we say about the color in icon graphics?
First, there is a certain rule: if the image looks poor when converted to monochrome, it means the colors that were selected are incorrect. This rule is true not only for icons but for the entire graphic design.
Second, you will probably unable to apply safe RGB. Sure, you can select GIF colors only, but this ties you hand and foot. Gradients, shadows, dithering and many other effects will not be accessible to you.
Third, try to increase the portion of "plain" color. The more consistently colored parts without blurring and jumps your project has, the more clear it appears. Pictograms are too tiny and the over usage of special filters makes the image look dirty.
Fourth, gradients. This is a wonderful instrument which can change the appearance of any composition. Other than esthetics, gradients are a perfect way to get rid of the "broken line" effect (lines with too long primary elements). But be attentive when using them because too much gradients can kill the plain color, and be unable to fit into the gif palette.
Fifth, shadows and flares. All rules for applying shades and highlights in icon making are completely the same to the rules of the overall graphic design. The only thing I would like to note: create everything by hand. Don't use effects, make all shades and flares in a separate layer and then edit the opacity. If applying filters, you rarely know what the result will be. It is bad when you can't control the process of creating of a composition. It is worse if you can't control it when drawing miniatures.
And, lastly, sixth. Tinting is the main aspect of icon creation, which largely results the esthetic appearance of the project. When the green human eye in the picture has the size of 1 pixel, increase the color intensiveness. For example, instead of 0, 131, 159 choose 71, 195, 242. It doesn't matter that the human eye can't have this color; in the first example it will appear like a gray dot, but in the second one it will really be a green eye.
Thoroughly control the image, do not allow the optical tricks to ruin your design. It is acceptable if something is not perfect from the technical point of view, but the whole composition must be flawless.
As an example, I pictured the packet of juice located in front of me. The image has plain color, gradients with broken lines, shadows and flares and nuances.
If I talked about something bigger than pictogram design, this chapter wouldn't fit into a whole book, but the resolution of images ;limits fonts too.
The size of letters is the main aspect as opposed to their look. Nearly in any case, unless the letters are the main part of the image, the font size has to be reduced to the largest extent possible.
In general, virtually any graphic company has its unique font with little characters. Such a font can be crafted in a few hours. You can browse the web and collect your own library of very nice fonts. Primarily, I would recommend you to download the DS Pixel and Seventen 7Vedi webpage and the whole series of fonts from the Lakmus Lab website.
If you don't need a new font, little typesizes of Arial and Verdana may do. As a last option, you can draw the necessary characters yourself.
There are quite a few rules there. First, characters cannot be less than 5 pixels high or less than 3 pixels wide.
Second, letters cannot use anti-aliasing. It turns the text into an unrecognizable mixture of pixels.
However, there are no rules without exceptions and the 5-point Arial appears perfectly readable.