Legal use aside....

In what format is the pic? What is the resolution? In order to print the image to the size you need it will have to be a HUGE file.

Check out online printing options, I've had backgrounds made before, and they're relatively cheap.

a good open source is sxc.hu Look for their highest resolution files.

...And when I sent my digital file to be printed, I was not asked if I had permission to print it. I have also never been asked if I had permission to print any pics that I have had professionally developed. IMO, while there may be some exceptions, I really don't think the people taking orders and processing them really care that much.