Ok, I wasn't clear. Let's try again.
1. I upload a picture
2. I edit the title, the desc and the tags through the admin panel (edit>albums>images) and save them.
3. Oh my god I made a mistake with the title, so I edit it (and only it) through the admin panel and save again.
4. Oh my god all the tags became weird : "this is a tag" became "this%2Bis%2Ba%2Btag" and this is the same thing for every images appearing previously on the edit page.
5. If I edit the tags directly on the image page of my theme (image.php) logged as an admin, I can edit the tags correctly without problems but as soon as I edit something (title, desc...) through the admin panel I get this problem with the tags of every image appearing on the edit page.
I wish I could be clearer.
Do you get it ?