No, its not ok. It's plagiarism. You're taking someone else's work, and while you aren't actually saying it, claiming it as your own. I've had a few of my sigs and avatars stolen by other people over the years, and nothing ticks me off more.
You definitely should contact him and ask him if you can use his work, and don't forget to either link to him or mention him as the original creator. This is very important!