Have you ever been asked that question, only to be stumped? With hundreds of gamer pictures on the Xbox Live Marketplace, I was constantly running into this problem when I get messages from the community. Fortunately, I was able to convince the Xbox.com developers to try and help. Now, when you are looking at someone’s gamer pic on Xbox.com (or in the Xbox forums) you can hover your mouse pointer over the picture and find out where it came from. While it won’t tell you exactly which picture pack it is from, it will tell you what title the picture is associated with. So at the very least, it will get you pointed in the right direction. Thanks to the big brains on our wonderful dev team for sneaking this in for me. One more thing: it may not work with every browser, but it should.
I uploaded a screencap to Flickr to show you the alt tag action, or you can just check it on your own gamercard.