I assume that a message with an image from someone I
> don't know is likely spam and bounce it with an appropriate error.
People who "bounce" messages only make the volume of traffic higher and contribute to the spam problem by sending their "helpful" bounces back to the forged sender address where the bounce is just more spam for the person who never actually sent the message. The universe would be a much nicer place if all bounces stopped happening :-).
It is particularly irresponsible that there are commercial products which are configured out of the box to send bounces (especially the anti-spam tools which recognize particular viruses known to forge mail headers and bounce them back to the "sender" anyway).