I personally store pictures within an "images" subfolder of the module folder and, depending on where/when I want to show them, I may use code to resolve the url -- simply ResolveUrl("~/DesktopModules/MyModule/images/someimage.gif") will return a string that is appropriately pathed to show your image.  On the same token, if you are using an ASP:Image or ASP:ImageButton you can/should use the "~/DesktopModules/MyModule/images/someimage.gif" and it will resolve correctly.  If you are using a standard <img..." tag then you will either need to put some script in there anyway, or try to use relative pathing ("../.." type, but I can't really remember).

Hopefully that helps.