The best size is around 150 by 150 pixels (this is the same as most of the pictures on the site). Most paint programs have the option to resize your working area. Try looking under the "View" or "Image" tabs on the toolbar. If you can't find it there then try searching the programs Help file for the word "resize." To make it easier for people to browse the pets, please make sure that the images you create are less than 75 kilobytes.