Good first plan! It actually will mean tons of work and now we are really few so IMO if you ahead with the idea i would suggest to start focusing on a single part first. For example, start by covering a small system with title / ingame shots. It would be better to have something (or a few of them) finished from time to time, that's the reason of my suggestion. Beginning working on all collections and various systems already will spread the work and take a long run until results are observed.
Other concerns are also things like:
format (png)
size (original resolution)
image names (how we plan to relate images to sets - in a db name is unneeded although having a standard, like use image_crc.png or variations is good. On the other hand, to provide images to others they may be able to relate images to sets, mame (you referred it) solves this with name, but they use 8.3 names)
images repetition - In TOSEC we have tons of sets that are "repeated", from different dumps of the same software (alts, hacks, cracks, bad, etc), various disks, to clones, different versions or localizations and such. Many may be exactly equal and so share the same pic, others may have small but noticeable differences and so different images. Base only on a naming scheme here would make it highly complicated, we could try to link images to set titles instead of the entire setfiles too.
All these and more should be well defined before start, as for the web thingy idea, it is something that takes tons of work too to produce given the amount of data and complexity of some things (like the convention), something that i'm really missing lately (and will be for a few more months at least). Anyway i have a similar idea but not that much time to explain now, i will try to explain later if i get the time, pm me!
