It's best if you use your own images or designs since they're yours and therefore AWESOME!
If you want to use an image from the internet, please keep in mind:
1) You have the right to use the image. (Sometimes we'll ask you to provide rights to the image. Sorry, it's the law!)
2) The image is high-resolution enough to make for a good print. (i.e. If it's good enough for Instagram, then it's good enough for #Yoshirt!)