I sync my Obsidian vault into my Android phone and recently got a bit annoyed as all the images that I had in my vault, kept popping up in my Gallery app albums.
I discovered that adding an empty file with a filename of .nomedia into the vault’s asset folder tells Android not to scan that folder for images. Here’s an article about the file and its usages.
There is also a plugin called Android nomedia that was recommended in discussion forums but I haven’t used it myself since I put all my assets into a single folder.