Ok well firstly, Spark doesn't directly control the ad content. However, you can still report an ad if you think it is inappropriate - instructions are here : www.sparkpeople.com/resource/motivation_ar
However, while I agree that images of junk foods on a "diet" website can seem counterproductive or annoying, I'm not sure they would fit the definition of "inappropriate" - but you can report it anyways and see what the Spark staff think.
Secondly, you could install ad-blocking software (I use the Chrome browser and their adblocker was a free add-on).
Thirdly - images of trigger foods are EVERYWHERE. You can turn your head away from the offending ad, only to see images of cookies and milk flying across your tv screen, or screaming up at you from the magazine you are reading. You leave the house to get away from it all, and there's the neon lights of the fast food restaurants and convenience stores beckoning you. The images on this site are the least of the pressure we encounter each day. Coping with such triggers is one of the biggest challenges that most of us face.... it is necessary to learn to deal with "unwanted imagery" because it really is everywhere, not just here.
And hey - get rid of those Oreos. If they are a trigger food for you, you'll alleviate a lot of stress by keeping them out of the cupboard. When you know they are Right There, the stress of temptation and resisting-temptation can be a MAJOR drag and sometimes an outright motivation-buster/binge-inducer. Do yourself a favor, make it easier on you - and let them go.