Softening the image with image processing can reduce file size, particularly at the highest compression ratios. Compression Best used on graphics with minimal gradients Compression is lossless and takes advantage of long stretches with the same value. When using GIF reduce the number of colors to make the file smaller. Dithering averages two colors to get an intermediate color. The Interlaced format is like the progressive JPG format in that a lower resolution view begin loading quickly, as the picture is downloading from the net.
Most banners use animated gifs.
File size is everything. Sites are defined by download times. It pays to experiment to get the smallest image files. Resample to reduce size of image, just don't change the HTML display dimensions. Always use browser safe colors for GIF images and save without dithering.