There's a tutorial on the forums on how to make girls. (I assume it explains how to add images) BUT i'll help you out anyways.
If it's a pre-made girl most of the work is done for you. All you have to do is find accompanying images and put them in the folder. Name your new pics sex,les,anal,group, BDSM, ect. followed by anything after that. (I use the cammy folder for refrence) Commonly modders on here will accompany a number to be sure not to overwrite any pre-existing images. So say you have 50 sex images already, but found an image that will work as well. Just rename your new image to sex51 and place it in the folder.
Quick refrence for what images MUST be named to show up in game: (Off the top of my head)
Sex
BDSM
Anal
Group
Beast
Les
Hope that helps!