I've have enjoyed creating bottle cap art for some years now. Most all my designs have been created by me, mouse drawn in Photoshop. Some I acquire on the internet as public domain and others are purchased with a commercial use license.
Many times I have a customer request for me to create a set of bottle cap art that is with a photo of a famous teen idol or copyright images from Disney, sports teams, etc. Many people take images from the internet and do this type of collage with them...
and sell them online. (Note: above image came from http://lextrading.info/shop/index.php?main_page=product_info&products_id=5230&language=en)
This is so easy to do yourself! I have put together a "How to..." PDF tutorial. This includes step by steps instructions with screen shots to be used with a FREE online photo editing program. You don't even need to download anything. Also included is a 1" circle template in a 4" x 6" .png file.
All this for only $5.00 and under, depending on which shop you purchase from. I have this available in my Art Fire Shop as well as my Etsy Shop.