Yes, unfortunately, you don't have access to that information in the forum post parsing code. SEO wise (which I assume is the reason you want to do this), I'm not sure that's a great thing to do since you'll probably have many images in the thread ending up with the same alt tag, which may or may not accurately describe the image. If you don't accurately know what the image actually is, I'd leave it up to the search engine to determine it itself.

Adrian