There’s no definitive rule or criterium, because the sexist essence of an image is a question of context : images, but also slogans (sometimes both) can carry a sexist stereotype.
A sexist image uses sexist stereotypes (feminine as well as masculine) to sell a product. It perpetuates this way the clichés that found patriarchal society : a man is strong, a woman is pretty and delicate, he likes car, she does the washing up.
Isolated parts of the body, the absence of a head, etc. contribute to reify the human body and make it a product, it deshumanizes the women presented.
The question of the link between nudity and the product is often raised. When a naked woman is shown to sell a car, the answer is clear, the ad is sexist. But what about an ad for underwear ? Then the position, the slogan, etc. matter. But it also seems that selling underwear is used by ad makers as an excuse : most clothes brands (like C&A, H&M) make advertising on underwear (being then "obliged" to show naked women) when they actually don’t make their sale a priority.
A sexist image makes common situations of violence against women : images of beaten up women, who play with this idea, are not rare.
A sexist image imposes physical norms and ways to behave : men and women are worth only when they are young and handsome (thin for the women, muscular for the men).
from Emmanuelle de Champs and Thomas Lancelot Viannais, Mix-Cité