Solution
The images imported to Digital Photo Professional Express will be saved in the application, not in the Camera Roll.
Caution
The JPEG and RAW images edited using Digital Photo Professional Express (.CR3) can be exported to the Camera Roll. When doing so, RAW images (.CR3) will become JPEG images.
If the Digital Photo Professional Express applicaion is deleted, the images imported into the application will also be deleted. Also, the images imported cannot be recovered.
When importing images to Digital Photo Professional Express, it is advised to save the images on a separate memory card or devices other than the iPad as a means for a backup.