This is along the lines of putting text on your images:---I'm telling you now, to do this only to a parasite whose author is purposely defying your requests to quit linking images--- and if you are willing to temporarily stoop to thier level in order to lighten the load on your site, then you could put profanity, demeaning sentences, or even childish namecalling!(i.e. the author of this site is a _____ who doesn't know his/her______ from his/her_______!) I do think the advertisment of your site is a better way of going with things, though. And althoughI'm sure that you have already thought of these ideas before you read this message, I mainly wrote it to tell you that you're not the only one who wants to make a parasites life a living you-know-what! Of course, you'd wan't to warn them or tell them that you will take some sort of action. You can even tell them what you are going to do, but I personally wouldn't tell them that I'd swap the images around, I'd only tell them to get thier linked images off of thier site, OR ELSE!!!
The trick that follows will more than likely cath them off guard. Again, I'd only do this if I already told them to take care of the linked images.[Here it goes] The key is TIMING. You want to do the swapping, or whatever around a holiday. The reason for this should be obvious, but if it's not then I guess I'll have to tell you. Chances are, the parasite authors have out of state family, or stuff like that, which make it very hard for them to update it immediately. The way I see it, if you want to do this, you want to make sure that it won't happen again, and therefore you either attemt to redirect the traffic, or insult the author, or the surfers.
Also, NEVER,(I'll say it again)NEVER, use the name-calling or profanity tactic AND say something like" by the way, you should go to this site!" Because when they see the many insults, and then a reference to another site, people might associate the two pages, and think that they are basically the same.