So i was just doing exactly that and came across that same problem..i love problem solving(i recently officially admitted and started majoring in programming as well as art..lol) and so when i came across this i tweaked around and found no solution. i was pretty new with texturing so i thought i just didnt know enough...All answers above i tried and no solution..i went back tried again and looked at the error again. i looked at * specifically. AND BINGO. that was the problem. wherever you save..make sure you dont just write one thing. put a dot then write something else then save. ex: "uvsnapshot.o1" whatever you want you want to make sure there is a dot followed by something else. FYI i signed up just to answer you question!lol