I think the index post is a brilliant idea.
Maybe if users posted in other communities, but had a 'badge' at the bottom of the post or a footer that said 'these images are free to be used in the HIVE community' - and then shoot that as a DM to you - you could 'archive' the posts under key word headings in weekly curation or fortnightly curation? Not a busy curation - just an archived list with subheadings.
It would be easier for me to index if they're in the community, that way I can just go through the posts there each week rather than trying to find them all and risk missing them. By all means people are welcome to post wherever, it would just miss out on them being in one place.
That's true. Perhaps they could drop their post under the last curation post if they want their images to be archived?
@minismallholding maybe encouraging people to use the tag #stockimages or #royaltyfree?