Is it sin when I masturbate to pictures of a girl I think looks nice in my class as long as I don't want to have sex with her and other things? Like men like often look at the breast and the butt of a girl. Is it wrong to masturbate to it as long as you don't image that you are having sex with her?
I think you are playing games with words. If you might claim that you are not wanting to have sex with this particular girl, but you are using her features to sexually stimulate yourself. If you aren't finding it sexually stimulating, then there would be no need for pictures. But the truth is that you are finding certain features arousing and you are using that to make it easier for to ejaculate. Thus, lust is involved even if you have limited yourself for the moment to only thinking about body parts.
You are trying to find out how close to sin you can get without crossing over the line. The proper attitude for Christians is get as far from sin as you can because you don't want to get caught. "For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, thefts, covetousness, wickedness, deceit, lewdness, an evil eye, blasphemy, pride, foolishness. All these evil things come from within and defile a man" (Mark 7:21-23).