oops!! My Image Sitemap is working perfectly, now that I fixed MY screw up! I have my product names set as '<b>Product Name</b> - Short description'. This what is passed to my image display function, and used for the 'alt' attribute (I don't bother with 'title' attribute as my java tooltip script would just have to remove it). I simply added 'strip_tags()' before creating the 'alt' attribute and the world is wonderful once again, all images now have an <image:caption>. In my defense, I never read anywhere that the 'alt' attribute cannot contain any html tags... aw heck, they say a little humility builds character... I must have a lot of character by now!
Thanks for the offer Oleg, just glad I didn't waste too much of your time.