If you store your images in the "Images" subfolder of the project (this folder is automatically created for you when you create a project) then they should be stored relative to the XPression project path. This means you can copy the project to another location, or even another drive and it will still find those images.
When you load a project XPression caches the images from disk in local memory, so even if you change the file on disk XPression may still refer to the copy it has cached in memory. Restarting XPression, or using the Menu Item Tools->Advanced->Flush Texture Cache should force it to reload the images from disk.
So, to summarize, you should be able to do what you want (have projects fully built in separate folders instead of having to replace files between games). Just make sure you store the images in the project subfolder.
#XPression