If you make the assumption that you know the folder name for the module and somebody isn't going to change it, you could use a relative url (assuming it is an ASP.NET image control -- something like image.ImageUrl = "~/DesktopModules/MODULEFOLDER/images/expand_blue.jpg")

I use the same technique on several modules.  Additionally, depending on which class you inherit from on your modules (I typically use UserModuleBase, uncertain on which class it is defined) you can use ResolveUrl("~/DesktopModules/MODULEFOLDER/images/expand_blue.jpg") to convert it to a path usable by other things.  Basically the same thing the ASP.NET image control does in the background anyway.

Bill's recommendation is the way that I do it for my modules.  (now...)