>IANAL, but in the US you'd need (a) rights to the image being used (e.g. shot yourself, in public) & (b) a damn good argument that your juxtaposition of their likeness and your words doesn't cause them monetary damages (e.g. in the form of lost revenue from reputation).

Well... "need" is, as I often say, quite a strong word.

Might makes right, and money talks, so if you don't want to bother getting the rights, or the person who you want to exploit can't fight back, why not?

Because anyone you want to exploit by definition either has enough money or visibility to avail themselves of legal redress?