A while ago I found this website called Poolga, where you can get backgrounds for free.
And it’s not one of those sketchy websites; it’s a simple, easy to use, no strings attached, filled with good graphic art, website.
Some of my favorites:
This design I originally saw on Threadless. And I own it, on a scoop neck t-shirt. And it’s fabulous. One of my favorite t-shirts, by far.
To get the high quality version follow the link (the image is linked to the page you need to visit) and click on download.
Some images are optimized for use with iPad Retina (vs iPad) or iPhone 4/5 (vs iPhone 3).