On forums that don't self-host uploaded images, I just usually don't post images any more. Most of the forums I visit self-host, except ETS doesn't.

If I need to get an image or some other files to/from a fellow forum member, I coordinate with them via PM, and then give them an (expiring) link to my personal cloud service for upload/download. I host this cloud service myself on my own servers (using NextCloud and Docker container software). I don't use commercial services like Dropbox due to security/privacy concerns.