Problem with character portrait
As mentioned, changes to a portrait are not retroactive. So if you post, your post uses whatever portrait you have currently selected, and if you change it, any new posts post-change will use the new one, and old ones will keep the old one.
As for it "reverting", I assume you mean posts you've made since changing it are using the wrong one? You might try clearing your browser's cache, as it may be pulling old local info rather than new info for some reason.