The only part that matters in the file name is the
Image Names that the game will recognize at the beginning of the image file's name.
Anything after the image name is seen as a new file and included in the list of available images.
It is not case sensitive and extra spaces are preserved but I have not tested how it works with special characters.
If the girl's folder has all of these files, they should all work:
profile.jpg
Profile.jpeg
Profile.gif
Profile.png
profilea.jpg
profile1.jpg
profile(.gif
prOFile(1).jpg
profile (1).jpg
profile.1.jpg
profile...jpg
profile_girl.jpg
The only special case is "preg" image type because that is also the prefix for all the pregnant variants.
If a file starts with "preg" and is not a pregnant variant, it must be followed by a space or any of these characters for the game to see it:
0123456789().,[]-
If you need to test if a file will work you can load the game and go to the gallery.
Choose a girl that you can easily find the folder for and use her folder to test the images.
If you leave the gallery and reenter, the game reloads the images in the folder.
Just change the files in her folder until they all work.
The file name is on top of the gallery and it should give you a message if the image does not work.