Body image is often thought to be social, but when you look at the genetic components of sexual reproduction you find that much of it becomes genetic. While society plays a direct role in deciding what body images are considered attractive, the desire to fit this is completely genetic.
We must first understand that the drive to breed is a genetic trait common in almost all organisms. This drive is one of the few unquestionable instinctual behaviors of biology, meaning everyone is genetically driven to want to breed.
This pressure is the primary driver for one's body image, but there is another element often missed by psychologists and society as a whole. Attraction is driven by genetics, what we are attracted to is part of the same genes that ensure we are sexually attracted to our own species.
We can see the obvious flaw in this when you consider how popular beastiality is in many cultures. Their genetic contributors to attraction are skewed to cause attraction to another species or they possess no attractions at all.
A drive to reproduce without the attraction to a specific species would naturally cause someone to want to have sexual relations with everything alive. In nature this probably caused a lot of cross breeding responsible for the variations in life, however for humans it has no beneficial effects.
Humans are now so removed from the other animals, through crossbreeding with nearby species ironically, that we have no compatible species available. We see a few other species of animals which have the same type of isolation.
Since attraction is a genetic trait we all have the image of sexual attraction in our genes, the image considered perfect by society. We see this ij other species, commonly they are attracted to specific colors or markings, things which we would consider cosmetic.
Yes, this does mean that our desires to appear a certain way are actually driven by our desire to find a mate. Essentially, we want to be the one the other people desire.