Need help organizing, locating images

First off, my wife's passion is creating digital art and her tool of chose has been android devices. She'll fill up the devices until they"ll no longer function due to lack of space. Unfortunately most of the devices are running old versions of android so installing any new file management app is a issue.
I'm trying to get a handle on this by uploading and moving files to the sd card where possible. On a Samsung running android 6.0.1 I've found images in the following files.
MyFiles/LocalStorage/DeviceStorage/Pictures (118)
MyFiles/Images (1.7GB)
MyFiles/Downloaded Apps/Photos ( 382 MB)
I've already relocated the DCIM file (441) to the sd card. I understand these pics are from the camera. But before I move on, can someone explain these files? I'm assuming one is tied to Google Photos but if that is the case why is there data in the file if the pics have been uploaded to the cloud?
How about the pics I found in the downloaded apps file, I'm assuming they can now safely be deleted and where do they go after downloading?
Are images automatically saved in both Goggle Photos and Samsung's Gallery and if so what determines which app is used?
Thanks for any help on this problem.