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I'm looking at it from an SEO point of view.

The site we have has been in operation for several years before I got it. It happens to have an affiliate program, where the images are hot-linked from our server, to the affiliates page. The affiliate gets the code from their login screen.

However, the alt attributes weren't set up very well from an SEO perspective from the beginning. I've fixed it since then, but would like all those other pages to load with the updated alt data.

I doubt this is feasible but worth asking.

I just started doing this.

Short answer: from just HTML, it can't be done.

But it's fairly easy using Perl or PHP, once you've added XMP data tags to the image itself. XMP - Extensible Metadata Platform - a metadata spec. by Adobe but an open standard, and growing in popularity, you can embed any amount of descriptors into most images. Typical XMP data includes title, description, keywords, creator, copyright, though you can add your own custom types of metadata, as well. Good intro at http://en.wikipedia.org/wiki/Extensible … a_Platform.

Image formats supported: JPEG, JPEG 2000, GIF, PNG, TIFF, and anything by Adobe. I use PixVue - it's free, and it doesn't clutter my images with metadata I don't want (Adobe's products, not surprisingly, do). Two clicks apply my standard template to all selected images, and then I edit the title, description, keyword tags manually.

Perl's Image::Exiftool handles XMP metadata, as does the PHP Metadata Toolkit (Google them), though the latter doesn't handle images created in Photoshop CS2. For me, that doesn't matter. I mainly use the PHP Metadata Toolkit to pull information from the images and deliver them to my pages. Or you could use such routines to draw this data from the image and log it to your database of choice.

FWIW, I suspect image metadata will become something that image browsers will look for (if they don't already), and that if you derive web traffic from Google Images or similar search engines, you'll get more hits from well-annotated images.

Final note: Exif metadata is old, inflexible, has idiosyncrasies, and works only with 24-bit TIFF and JPG. IPTC metadata may be used, but is not extensible, and IPTC itself has embraced the XMP standard ...