TLDR: Create a file called .nomedia in the directory of which pictures you don’t want to see in you Gallery app.
I have a lot of ebooks on my phone, it syncs the page status with my ereader so when my wife and me go shopping, I can just join the ‘dad section’ of whatever store we’re at and dive back in to my book(s)
But having so much e books means that they all appear when I scroll through my pictures.., especially since they are all located in separate folders per author and per book (yay for ordering!)
Unfortunately so much folders mean that each folder is individually shown in the gallery app when browsing pics. Adding a file called .nomedia means it hides all pics from the gallery from that folder (and it’s subfolders!)