If you're looking especially for Team GB medallists, you can start with this slideshow of the UK gold medallists (29 pictures in all). You can also view photos of all gold, silver and bronze Team GB medallists here. You can also get a GB medalllist's album
For other photos, you would have to do a specific search. For example, you could look for 'russian gold medallists' (plus the name of whatever sport you're interested in) and click the 'images' tab, which will give you a choice of photos. You can't get one photo of all the world's medallists because there wasn't one taken - if you think of the way the Olympics works, people are photographed winning at different sports or in their teams, and again you can get shots of all the medallists of a particular country (usually in the form of an online gallery) but you don't get all the world's winners lined up together. However, if you do a search along the lines I've suggested, you should find plenty of images.