On PSE it is common for users to create puzzles using photographs or pictures taken from elsewhere on the web (e.g. here and here). Now, I like and enjoy this style of puzzle - it challenges your brain in different ways to a word-based puzzle, and I would like to create a puzzle of this variety myself. My question is this...
What is the situation of an image-based puzzle with regards to copyright? I am no expert on the law or principles of the Web when it comes to this, but if the images required are of people or things that the OP cannot easily photograph or draw themselves (e.g. a celebrity headshot, a deep-space image of a planet, or a scientific diagram), is there anything in the PSE rules/guidelines which requires attribution/acknowledgement of some kind to be made to the originator/source of the image? I have never seen such acknowledgement made on a puzzle, which makes me suspect either there is no such requirement or all of the images used have come from some kind of online image repository which permits their usage here. (Or that there is such a policy but nobody bothers to adhere to it!)
Ultimately, my question boils down to the specifics of: Are question askers (and answerers) free to use whatever images they like from elsewhere (the 'because it's available on the Web' argument), or are there only certain online image repositories which permit their images to be used in this way?
A naive question perhaps (and one which I am sure affects far more websites than just PSE, so may well have a Web-wide policy solution), but one which would be useful to have some clarity over...