Partially related to changing images/gifs that have been posted as messages, though I'm suggesting a different solution to a bigger problem.
When creating a forum post, a name can be given along with an initial description. The original post can be edited to change the title, but the image selected initially is given as an embedded part of the description post. Because of this, the image can no longer be changed after the fact, even though the title can.
In the server I use forum posts, the image is a preview of the project discussed. This however means the image becomes increasingly outdated as the project progresses.
Solution: Make the image/gif an integral part of the post, essentially making it part of the title. This to enable the image/gif to be modified/edited the same way the title can.