All I can say is that mostly when I come across profiles with background images when I'm sourcing or data doctoring, the background image is so busy that it makes it hard to look at the page in display mode, which usually means that I move on to work on something else.
I'm not totally opposed to background images; I have used them on a few occasions, taking care not to disturb the eyes of people at my own level of over-sensitivity. I guess I wouldn't mind removing them if they hurt the eyes of someone else.
This, of course, does not address your case of an inappropriately chosen background image.
I haven't checked the help pages to see if they address the correctness of background images - although from G2G discussions I think there is an agreement that incorrectly applied coats of arms should be removed, as also pictures violating copyright should be removed.
Also, I am sure there is nothing that says there MUST be a background image.
As for protocol, I guess the usual recommendations of politely contacting the profile manager are valid. If the manager is long-gone, well, then they cannot be contacted. It's the usual scheme of allowing time for communication, in accordance with the situation.