To me this sounds dodgy. The whole point about these ads is that they are supposed to be unintrusive and not have images.
Abusing HTML to re-inject what is in effect just that, images, sounds like blatant violation of TOS. This can cause bad rep for the ad-provider who has promised site-owners image-free ads, and bad rep for site-owners who has promised end-users (or maybe VIP members) an ad-free or ad-limited experience.
While this may technically be a "cool" (yet obvious) hack, the reasons stated for doing them are wrong on every single level. I wouldn't want to encourage this.