With the current way the game works changing the game's system folder is difficult. The simple way is to create a symbolic link of folders using the OS.
Otherwise, the game should support changing the portrait and body image folders, though it's a bit bugged in 0.5.24c. I have a bugfix on Discord that should fix it.
You will need to create folders for "portraits" and "bodies" in the D drive. The folder select window is found by selecting a slave, Customization tab, Portrait/"Full Body Image" button, and Select Folders button.
Note, if when selecting a folder for your images the button says "Select Current Folder" then the Path will be used, if the button says "Select This Folder" then Path + the selected folder will be used.