I agree with many other posters here. Your butt looks fine. I really don't know what the problem is. I can't see any cellulite either but that may be because of the lighting and angle. I guess you just want your thighs/sides of your butt (the part right below your hips) a little slimmer? If that's the case, you may or you may not be able to do that. You just may genetically be "cursed" with that sort of butt. Again, I don't know, but you should try some of the suggestions others are offering here and see what happens. I would say though that even if your butt looked the same 6 months later after trying everything, you're not bad off. Plenty of guys would like what you have.
As for cellulite, sorry, but there's no way to get rid of that. There MIGHT be some sort of surgery that can reduce or maybe even get rid of it entirely, but that's obviously not cheap and, again, I'm not sure if it would really work. Besides that, cellulite is something a woman is stuck with, even if she gets very lean from working out and dieting. What being lean WILL do though is make the cellulite less noticeable. I didn't even see any in the picture, so, again, you're not bad off.