It is possible to convert synced photos to camera roll.
The Apple Community post "Syncing" Photos from iTunes into Camera Roll shows how.
Note that duplicating a large number of images will take a long time, I had something like 500 images and it took at least 10 minutes.
This method is the fastest way to do this that I know of. Note that creating an album out of your synced folder does not do the same thing as duplicating these pictures. Even if you have a computer with iTunes installed available, I was not able to find a way to get access to the synced photo data in iTunes, and synced folders are not shown in the DCIM folder.
From what I have read there are some third party programs that might be able to help with this as well, but I was looking for a builtin solution.
Process for "duplicating" a synced image folder to Camera Roll:
- Open the photos app and open the synced photo folder (it might be under the category "From My Mac").
- In the top right corner, hit select
- Select the images you want to "convert" to camera roll images (or use Select All in the upper left corner)
- Hit the button that looks like a square with an up arrow coming out of it (in the lower left corner), a menu will show on the bottom of the phone
- On the lower scroll bar, scroll over until you see a "Duplicate" button, if you press that your iPhone will copy the photo to your camera roll.
Photos that are transferred using this method are then treated as normal Camera Roll pictures.
Moving synced images to a non-camera roll album
If you would prefer to put the images in an album other than Camera Roll when duplicating them, you can add your synced folder's images to a new album and then duplicate them; all of the duplicated images will be placed in that album instead of being put in Camera Roll.
To do this, select all of your synced images (Select -> Select All), then click "Add To" and you can then add them to an existing album or a new album.
Then, duplicate all of these images using the same steps shown above for the Camera Roll.
After this process is done,
- You will now be able to connect your phone to your computer and copy the pictures out of the DCIM folder (which is what I did) without having to worry about losing any previously synced folders that you don't have access to anymore.
- You will be able to delete these photos without doing a sync
I did this on iOS 12, in case this behavior changes on later versions of iOS.