So, if I understand you correctly, you want files (pictures) that are put into a specific folder to automatically get assigned to a specific group (photography).
A possible solution is setting the "setgid permission" on the parent folder, where all images are stored. The setgid permission (see e.g. wikipedia) ensures that all files and folders that are created in a folder with the setgid bit set obtain the group of that folder (instead of the group set by the file creator).
Since this does not effect files and folders that are already created inside this directory, first change their group recursively using
chgrp photography -R <your shotwell photo folder>
Now set the setgid bit on that folder. To do so, run
chmod g+s <your shotwell photo folder>
After both commands, your shotwell photo folder and all its contents belong to the group "photography", and the setgid is set:
ls -ld <your shotwell photo folder>
drwxrwsr-x 2 <your user> photography 4096 Aug 27 20:42 <your shotwell photo folder>
Notice the s in
drwxrwsr-x. That "s" indicates that the setgid bit is set.
Now each time photos are copied into your shotwell folder, they are automatically assigned to group "photography".
If files and folders are moved into that directory (instead of copied). Their group is not changed. This is because of the tool itself. "mv" basically just renames the file, keeping its original ownerships and permissions, while "cp" creates a new file in the target directory.