Single post in Regarding Art Theft

So since my question seems to have been missed earlier and Garthic's post clarified somethings but not this one very much: Example: Before I had a picture of a mew eating a cookie. It's not mine, but it had a signature on it. It came from tumblr and I can't figure out the signature enough to find it on deviant and locate its true creator. to avoid trouble though I had put below it a disclaimer like the following: DISCLAIMER: This picture is not mine. Credit goes to its creator whose signature is on the work. I claim no ownership to this work. Would that be allowed in this case? Since obviously it was marked, but I couldn't discern the name in the mark or find it on deviant and thus disclaimed all ownership? In other words if you add a disclaimer would you be allowed to use the works of art of which you can't trace the original creator? Also now I use my own work do I still need to have a the consent thing there? In other words how does one prove one truly made the work of art? On another site I had a lot of issues with this as people would start arguing with me over whether or not I really owned that deviant account. It was long annoying and pointless and I'd like to avoid any trouble.