Make it small file size in Kb. Speed is your concern Some banners exchange have a limit in size most often is 10Kb

Try to change resolution of your display to see how your banner will look under 256 colors, even under 16 colors. Switch your monitor to gray scale and look at your ad again When it looks right upload to your banner exchange network

Proportion [ In banners ]

Use mathematical principle to concentrate on size of elements. The idea is that equal elements proportions are visually boring because they are monotonous. Two part of the same size will fight for attention and neither one provides a point of interest. Try not to divide your banner in half.