I figured out how to do it on my Galaxy Tab A.
1) click on "My Files" on the main screen with all the icons
2) Select the device or SD storage where the photo album is saved to
3) Select the photo album that contains the photos you want to rearrange
4) Press and hold the picture you want to start with and a box will appear (showing you selected that photo)
5) Select "MORE" in the upper right corner
6) Select "Rename" - this is where you can label the photos individually, one at a time, in the order you want them 1-10 (65, 100, etc)
7) Back completely out of "My Files" when done renaming photos
Voile - your photos are now arranged in the order you want them.No need to download any unneeded apps wasting space on your tablet.
I hope this is helpful to you. I had risque photos that I didn't want showing as rhe album cover photo and had to arrange another photo in it's place lol.