%
Didn't realize that the tags would mess up the photo, too.
For the most part, however, I've been telling people to rename their albums and photos so that there are NO spaces anywhere in the URL. For URLs, spaces are changed to a "%20," and then forum is messing up the percent sign by converting it to HTML, which is "%", breaking the link to the photo.
But if you have a percent sign in your tags, you could put a percent sign at the start of your post. From what I've seen, the script only messes up the first % sign it finds in each post. For example, I put one at the start of this post to make sure that the other ones would be untouched. hth