Use sites like Wikimedia where it says whether or not it's available for use; though I think specifically just don't use someone else's personal stuff really.
You'll need to have the picture available online somewhere as the forum doesn't support avatar uploading (I think: I may be wrong, but I didn't see that option when I just checked) so it'll need to be hosted somewhere accessible from the interweb at large. Last I looked, Imgur was still a popular option for that sort of thing. The main thing is not to link to someone else's website as it can eat into their bandwidth allowance; and since its their site they can change the picture without your knowledge (I'm aware of some, erm, "hilarious" examples of that happening in order to prove a point!)
