For the definition below, I supposed you already know about Photoshop and digital image manipulation.
A "morpher" is someone using Photoshop in order to digitally manipulate images to change the shape of the represented objects for a given purpose. Here, the morpher digitally increase the size of females breasts so they appear more attractive to the average straight male.
Personally, I don't like morphers and I prefer digitally untouched images of females.