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Should I put a signature, watermark or copyright notice on my art before submitting it?
Signatures, watermarks and copyright notices are not required but their presence can help deter viewers from using your work without your permission.
By signing your work prominently, placing a watermark which is difficult to remove and by placing a copyright notice directly on your work you can discourage copyright infringement by those who view your work.
On the other hand, watermarks can significantly degrade the viewing experience. An alternative is to post in a small size such as 1200x1200 pixels that would not produce a good version for copying.