The test of "would you be comfortable with this slide being used in a 6 o'clock news story about the conference" would probably also work. A problem with these big arguments about talk content is they can become the only (or the main) thing people hear about the conference. Far better to leave out the unprofessional pictures (and language) in the first place.
I don't recall seeing similarly controversial content in talks at academic conferences I've been to (e.g. OOPSLA) -- is there something different about the free software world that encourages this sort of thing?