How am I going to figure out whether an image is sexual or not, if I'm not supposed to look at them?
If an image gets you sexually stimulated, such as a odd feeling in your gut or your penis getting erect, then it is likely sexual in some way, but when you are first developing, your signals may be unreliable. Erections can occur, even when you are not being sexually stimulated by an image.
Images that focus the sexual parts of the body, such as the breasts or groin in girls, are designed to stimulate the viewer and so should be avoided. Images where the person wears little clothing or clothing that is too short or doesn't cover fully would also be in appropriate. Even though the genitals may not be directly seen, it causes the view to imagine what is not quite seen.
Images that depict sexual acts would be wrong. Thus, a picture of a man on top of a woman where it appears their pants are down would be wrong. Pictures of sexual touching would also be wrong, such as hands underneath clothing.