entry, using a single still can constitute infringement though it's usually not likely to result in a lawsuit. The concert film is most likely protected under copyright law as the Who haven't been around long enough to trigger any of the traps that would drop the movie into the public domain -- for example, if it was published before 1964 and not renewed.
Fair Use Dept. It sounds as if you have the makings of a decent fair use argument (although we'd prefer if you didn't get embroiled in that type of dispute). Here's a link to compare your use to other visual arts fair use cases.
Staying out of trouble dept. Are you selling only a single print? Problems are more likely to develop if you market the work in bulk. So if you're paranoid about being hassled, don't mass produce your prints.