it could be a great and efficient feature too
I'd suppose there are many technologies on the market, which suggest image sharing, and each user could suggest thousands of features which s/he considers "efficient"; but that doesn't mean they would actually work for the major crowd.
What differs miniBB add-ons like File Attachments
or File Bank
from them, is that you upload images directly
on your own server and do not host them on 3rd party servers. In some cases, this is extremely useful for closed, adult or original content which could be just forbidden on 3rd party side. I'm more oriented to this, because I myself would not share the original content with 3rd parties and that way giving them profit from my pocket.
I'm not sure what's so significant in imgur comparing to let's say photobucket or imageshack services, and why we would need APIs for them embedded into miniBB? You may upload files to such services using lots of other tools. Then just link those pictures on forum using the standard BB code, and voila. Nothing could be more simple and free than this.
In terms of Paid Support
I could implement whatever new suggestion comes in (it may be for your forum only); but sorry, I could not handle them all for free on my own. I am working for miniBB, for myself, but not other companies whatever big names they could have. The same all major miniBB customers do.
Additionally to that, it's out of my own interest to develop something which connects to 3rd party, because it would require instant watching and updates. Comparing to static add-ons we have directly for miniBB, they require just a few updates per years.