How much depends on which 256 colors get chosen and how they are arranged in the image. However, producing images for the web adds an additional complication. If you care about these people, you ll have to take this restriction into account. Image editing and manipulation tools like ImageReady know how to deal with these restrictions if you need to work within them. Because GIF provides you at most 256 colors to work with, the types of images best stored in GIF format, and also compress the best, are ones with few colors.
Examples are a simple arrow of one color or some graphics headline type.
Other items that work well are vector graphics that have been converted to an image. The colors tend to be bold and there are large areas of even color. The larger the areas of constant color, the better the data compression portion of GIF can work. The choices you have to work with in a sophisticated image editing program designed for web work are in areas of how to choose the set of colors you will actually remember for the image and how to represent colors that you can t remember. For instance, think of the problem of how to choose the color for a pixel that has a black neighbor on one side and a white one on the other.