The entries for the images should be located directly below the blueprint permissions.
The path to the permissions should look like this:
Server root directory > Permissions folder
This contains two subfolders for Areas and Groups, as well as the default.json file.
I assume you're an admin in an admin group, so you should edit the admin.json file (in the group folder; you can do this with any text editor).
Game updates do not change the saved permissions.
Entries for images are (again, -1 for unlimited):
"image": {
"delete": true,
"deleteown": true,
"maxamount": -1,
"maxresolution": 1024