Currently I'm working on a similar tool. The idea is that I load all images in a single file. I have the whole thing nicely organized in a directory like structure, so I can have a toplevel, sublevels and the items (sound/images) within.

In my game I'm able to load portions of this file using myFileStream.seek(Offset, Origin) and myFileStream.ReadBuffer(buffer, count); So there's really no need to load the entire file.

There have been a couple threads about this particular topic in the past though.