I tried to use one of those crossword puzzle images to boost but it got declined by Facebook because the text was over the 20% rule.
Great question, I would think that Facebook has algorithms and people on tasks to review this simultaneously. Your question regards more so "hacking" into their platform to just promote a picture. Don't know why you are trying to do this, but maybe find another way to promote the pic, such as tweeting it and then promoting that text on facebook or something.
