"(such as having users upload their images directly into their posts vs using 3rd party services, and forcing them to not have images in their signatures and to upload their avatars to your site vs hosting externally)"
I don't know of any way the users can upload images directly into their posts. I'd love to be able to force people to upload their pictures, storage space is relatively cheap.
File attachments. Enable your users access to Attachment Manager. Its been part of the UBB.threads software for over a decade.
Per forum / group:
Control Panel > Permissions > Forum/Group: Max file attachments
Control Panel > Permissions > Forum/Group: Max file attachment size
Image attachments can further be processed within:
Control Panel > Master Settings > Attachments
When a user has access to attach files to their posts, they will see the "Attachment Manager" link in the Post Option section of their post.
See attached images