[Suicide Junkie]

Aside from the size, there is no need to worry about conflicts with system images, since they have arbitrary filenames.
If it became a problem, we could just have modders put their own name or their mod's name in front of the regular filename.
EG: Proportions_starmap.bmp, TNGmod_starmap.bmp, etc.

For the imagemod images, they all need to be given their number and there is a components.bmp (and facilities.bmp, and etc) which must contain all the images you want to use...
Having a single file makes things difficult for compatibility, and you end up having similar almost-copies of the same file in each mod folder. Thus the reason for the imagemod.
Make a standard all-inclusive image file, that all mods can use, cutting down on the download time for individual mods, and reducing harddrive space usage.