The Million Dollar Homepage guy is doing it to avoid student debt.
"The idea is simple: to try and make $1m (US) by selling 1,000,000 pixels for $1 each. Hence, 'The Million Dollar Homepage". The main motivation for doing this is to pay for my degree studies, because I don't like the idea of graduating with a huge student debt. I know people who are paying off student loans 15-20 years after they graduated. Not a nice thought!"