Okay, so here's the thing...
Are you violating copyright by using images you didn't make?
The short answer is yes. After a quick search on etsy for bottlecap images, here are a few people in violation of copyright: bieber fever Dora Mulan Princess Tionna Strawberry Shortcake Hello, Jacob Vintage Photos
If you did not take the picture yourself, use a royalty free image (you can purchase these on sites like istockphoto, but even then--they usually have limited uses for things like this, or draw it (digitally or by hand) then you are breaking a copyright law by selling those images even though you cleaned them up and made them all awesome for your bottlecap site. (That means if you are just doing a google/bing/flickr search for princess images and then using the best ones--then you didn't make it yourself)
Here's an example of someone who (I think) is not in violation of copyright: Christmas Images
It would be like if I went in, took pictures off your personal blog, and turned them into bottlecap images--even though I digitally enhanced them or whatever--it would still be your original photo that I used, and it would *technically* be illegal for me to use it and make money off of it without your permission.
That being said--are the Disney/Nickelodeon police going to come knocking at your door and demand payment/throw you in copyright jail?
Yeah... probably not.