When using other artist's work please be sure to read and abide by their personal TOU's. The majority of artist's that do give permission for use of their artwork do specify that you do not use their work in collaboration with another artist's work.
A major misconception to copyright is that if it has not got a copyright mark on it then it is not copyrighted, and one is free to take and use it anyway they wish...
This is not so copyright does not cover an idea in one's head but once it is tangible form it automatically becomes copyrighted material. Everything found on the internet is copyrighted. One does not have to register a copyright for it to become copyrighted.
Do not go on heresay from a website or someone telling you that something is okay to use. Unless you know yourself where it comes from and the origional artist it is best to follow the "Golden Rule" If you do not know for certain "Do Not Use It."
Many take other artist's work and change a few things then have the audacity too offer it on their site for download claiming to be the copyright holder. "BEWARE."
If you do have material that you would like to use but are unsure of the artist or if permission is granted for usage.
Please post a sample of the material you wish to use to the "Copyright Forum." Please mark across the material you are uploading with "For ID Only."
We will do our best to help you find the artist. To gain permission from the artist if they do allow usage is up to you to contact them for permission.
With that said a lot of material that is used for 2D programs such as tubes, stamps etc. have been scanned from magazines, books, greeting cards and are hard to track down.
Public Domain Material
Not all offered on these sites for download are copyright free so "BEWARE."
You are still responsible for checking the "Copyright" on the material.
Moderator's of the Copyright Forum
"The best and most beautiful things in the world cannot be seen or even touched they must be felt with the heart" ~Helen Keller~