Signing a Copy of Someone Else's Painting or Photo?

fancy pen signing a name
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"Most of us new to the art of painting start by copying paintings we find in photos or books or over the internet. Sometimes these paintings are very good. Can we sign the painting with our own name or not?" -- Sam E."

"I don't have a lot of knowledge about painting. Because of that, I find that I can do the best painting I can by finding a picture of a painting and copying it. I asked about entering my paintings for a local art show and was told I should put a note on the back of the painting saying that it was not an original painting, only a copy of an original." -- Pat A

No matter how good a copy it is, it remains a copy. Yes, everyone makes copies while learning to paint, but doing it for personal study and development falls within "fair use". Selling it or exhibiting is something else. No matter how proud you are of the painting, it's not your original creation, it's a copy.
If you did add your signature you'd probably want to be very clear about it being a copy and not an original because the latter is heading into fraud territory. Rather leave it unsigned, in your portfolio, and wait until you're painting your own original compositions before adding your signature. See also: What about Paintings Made from How-To Books?
If a painting is out of copyright, it's in the public domain and you are free to copy it, though you wouldn't sign it as if it were an original painting because it's not. Making a painting of an artwork or photo that is still in copyright is a completely different thing. The copyright holder of the image holds the rights to the making of derivatives.

Disclaimer: The information given here is based on US copyright law and is given for guidance only; you're advised to consult a copyright lawyer on copyright issues.