Well, that was a flat bust. Drag and drop doesn't do a thing.
However, This screenshot from iTunes on my Mac seems to have the answer.
Just select the folder where the photographs are held on your PC and check the sync box. I'd do a backup of the iPhone and transfer any photographs already on the phone first in case syncing wipes out what's already there. I never use auto sync on iTunes. I prefer to control what's happening.
View attachment 28479