Selling a photo isn't really prostitution in the way people usually use it. No physical contact, no risk of STDs, no commitment to anons purchasing, it doesn't count.

If you put a lot of time into it or doing it more long-term or they're expecting you to be somewhere and you're gonna be late, sure. If it's a one-time thing that takes a very short amount of time, it's insignificant enough to not matter whether or not you mention it. That's not hiding anything more than not telling them you tied your shoes that day is hiding something.