Is it saving full copies on the internal?

Crappy gallery (and other poor apps) rely on thumbnail images to guide the user and sometimes themselves.

(A thumbnail is a very low resolution 'copy' of a picture.)

Often these can build up and a folder of thumbnails can become quite large.

The other thing is that they are not particularly easy to find, as they are in a 'hidden' folder and can be in ANY app that deals with media in any way.

Each app can also have its own thumbnail, so you can even have multiple copies of the same thing.

There is a way around it, but it takes some doing.
You may need to change your Gallery and/or File Manager apps, or possibly more.

Finding them is done by first opening your file manager, enabling 'show hidden files', and then start with the DCIM folder.

If you see a folder called .thumbnails (notice the period before the word) check to see how large it is.

Also check around the other folders for anything that has a .thumbnails folder in it.

Start there and see if that is the issue, and then I can tell you how to be rid of it.

 

Yeah, your PC probably can't see them because of the hidden file status.

You can delete the .thumbnails folder on the phone, but it will be recreated and then grow again.

To end this problem, you must delete the FOLDER called .thumbnails, and then create a FILE called .thumbnails.
(Be sure that this new file has the exact same name and is in the same place as the old folder.)

The dot before the name is important, as is the creation of the FILE instead of a FOLDER.

Android will not allow two things of the same name to exist in the same place.
The device wants to build a folder called .thumbnails, but it is not there and a file by that name already exists.

So in this way a new folder is blocked from being recreated, and because it is a file instead of a folder, it will not be added to by the app using it.

One issue with this is that cleaning apps often will clear thumbnails, and they will also clear your file.

So then the device will once again build a thumbnails folder, and it will grow.

Either don't use cleaning apps, or use one that allows you to add things to a whitelist- then add your file to that whitelist.

 

I dont know what file manager you use, and stock utility apps are notoriously junk.

Here is a link to an exceptional one that is self explanitory, and will make everything easier.

In fact, if you like it, then the entire catalog of apps from the same developer is great and very small- they are effiecient and work together very nice.

https://f-droid.org/en/packages/com.simplemobiletools.filemanager.pro/

https://www.simplemobiletools.com/

The entire catalog of pro (paid) apps is svailable for free on F-Droid.

This is totally legal and supported by the developer (he placed the apps onto F-Droid himself).

 

To be sure of things- because I dont want you to lose anything important, make sure that all media (photos, videos, documents, etc.) are on a SD card.

The file manager app and gallery app from Simple Mobile Tools are great for this.

https://f-droid.org/en/packages/com.simplemobiletools.gallery.pro/

Get Simple Gallery Pro, and get into the settings.
Near the begining of the settings is 'Show hidden files'.
Be sure to turn that on, then back to the app.

My thinking here is that you have an app with a large cache of images.

These can be thumbnails for an app, icons, or any number of things.

Things inside a cache can always be deleted.

If you find such a thing, it can be deleted with the gallery or file manager app.

 

Is there a folder (in Gallery app) that is titled Pictures or such and appears all black?

When I move pictures from internal to the SD card, I often see that there is a remnant left behind- just a black nothing with a title but just all black when I try to view it.

After verifying that there is nothing there, I delete them.

 

And one more app to try is CPU Info.

It will tell you all sorts of things, including the amount of storage used on the device and the SD card.

https://f-droid.org/en/packages/com.kgurgul.cpuinfo/

 

Do you have any photo/video editing apps?

These can make copies of your media for themselves.

Finally, I do have an old cleaning app, and I keep it mostly because it does find duplicates.

It can find them and delete them.
You can even see where they are being stored, so you can figure out what app is causing the duplicates.

This is one cleaner that (if you use the cleaning part as well) that will erase your .thumbnails file unless you uncheck it first before you clean (it will ask you, and give you a chance to see what it is going to delete).

This is a very old version, before it had ads- so you do not want to update it.

https://systweak-android-cleaner.en.uptodown.com/android/download/1633705