I'm at 24k users for my little website. many of the posts have attachments. Photos of custom cars and car parts.
Use the groups settings to enable certain groups to upload images for their posts.
If you're looking for something outside a discussion forum, have a look at a gallery software. Codex Gallery2 had worked for me for several years. I gave certain members their own gallery and hosted their images.
PhotoPost was another one, back in the day. But i wouldnt do business with them today. Plus, they havent updated their software to use any of today's standard.http://www.photobucket.com/
is what most other users used instead. Let someone else pay for the bandwidth - am i right?
Anyways, whatever you choose to do with your forums, lets us know. It would be nice to hear what you come up with in "just a couple of evenings work."