I have tried a dozen times, resetting my computer in between attempts, but it won’t let me save additions to the bestiary.
I make a monster, name it and add the picture. Close the bestiary, open it and the monsters still there. I save my campaign, close DMHelper and reopen it and my monster is not in the bestiary anymore. Again I’ve reset my computer between attempts and have saved multiple times both through the file menu and with Crtl S.
In my current homebrew campaign I can’t really be making 4-5 homebrew monsters before every session only to have to do it again next week.