Every design you upload must be your original work, or something you have the legal right to use.
You're responsible for all the content you upload, so avoid using copyrighted material without permission. Content is generally okay to use if it's:
Your own original work
Something you have clear, written permission to use
In the public domain
Covered by fair use
Fair use depends on a few things: how transformative your use is, the nature of the original work, how much you use, and whether it affects the original's market. When in doubt, check with a legal professional before uploading.
CO:CREATE doesn't take on legal responsibility for infringement claims, so you're responsible for making sure any third-party content you use is legal.
For the full details, see our Terms of Service.