http://upload.wikimedia.org/wikipedia/commons/b/bd/Zytglogge_clockface_detail.jpg

He's offering to pay $50/hr for the winning submission, but that time includes working with him to make some adjustments if he wants them.

But presumably, you'd get your modified image on the cover of a physics book, which is better than the money.

P.S. The cover is due this week, which is why the prof is desperately seeking student help.

Addendum:

For the record, I did not ask the professor if I could seek submissions from Hacker News, but I would presume it doesn't matter where they come from.

Also, this is not an explicit promise than anybody on here will get paid anything; it's just relying on trust.

If people could just post links to their submissions in the comments, that would be ideal.