The way permissions storage and queries are handled in completely different since OpenACS 4.6.1. It is not comparable to ACS 4.2 at all. Take a look at this:
https://openacs.org/projects/openacs/proposals/scalability/permissionsYou could save months of work implementing your image repository using the OpenACS content repository. Greenpeace International uses a content-repository based system to manage the image library on their web site, although I am not sure how many images are in there.
Besides that, I think you might find some other interested parties to building such a system for OpenACS.
