I wonder why orgasm is so often associated with human female images.

I wonder too, probably because female orgasm is more intense than mans. Even some scientists say that if man could experience such a powerful orgasm, their brain would "explode" and they would be dead of course
http://notelay.com/articles/did_you_know/female_orgasm_deadly_for_men/

No direct connection or correlation was ever proved between the relative strength of nerve impulses and the intensity of a specific experience being reported. Therefore, is seems we are dealing with a myth rather than fact. If to judge the intensity of pleasure by the relative strength of motivation to gain such pleasure, it seems that males and females are more or less equally motivated to have orgasms. So perhaps the experience is not that different after all.

orgasmic