for the search for new volunteers I would like to use a stock photo. It would be on paper with some text hung up at some stores, animal shelters etc. as well as postet on some social media sites. It would reach approx. 3k people (hopefully more), but obviously no profit made. It's for catching the attention of people.
Is this allowed and which license would I need?
You should be fine with the standard license assuming the volunteering positions don't go against the terms. I would include a small image credit on the poster though.