When you are designing and producing graphics for the Web, it is of utmost importance to keep your file sizes as small as possible. The standard guideline for estimating download time over a modem is 1 second per kilobyte. Of course, actual download times will vary widely, but it at least gives you a ballpark number to use for comparisons.
There are a few simple strategies you can follow to minimize the size of your GIF files.
You can help keep file size under control by the design decisions you make. After a while, designing for the Web becomes second nature.
Though it may seem obvious, the easiest way to keep file size down is to limit the dimensions of your graphic. There aren’t any numerical guidelines here; just don’t make graphics larger than they need to be.
You can make design decisions that will take advantage of the way GIF compresses files. If you design your graphics with flat color from the beginning, you are basically giving the LZW compression the kind of file it likes—rows of repetitive pixel colors.
Fill areas with solid colors rather than gradients (fades from one color to another).
Limit the amount of photographic material in your GIFs (use JPEGs for photo images).
Favor horizontal fields of color ...