A .nomedia file is a blank file placed inside a folder. When the media scanner detects the .nomedia file, the media scanner does not scan that folder so the images in that folder don't appear in the Gallery app. It's a good way of having a folder full of images (like a skin for an app) and not have those images appear in the Gallery app. You can place a .nomedia file in any folder but the filename must begin with a period. Also, on a Linux system a "dotfile" (a file with a filename that begins with a period) is hidden from standard view, though most file managers have an option to allow viewing dotfiles. So the file is hidden from view and the images in that folder aren't cataloged in the Gallery app.
- Piktures doesn't respect .nomedia file if it is located in the / folder. Otherwise, a simple .nomedia file would make all photos invisible.
- Piktures doesn't respect .nomedia file in the /sdcard/Android/data folder because many folders are typically located there and respecting it would also make many photos invisible.
If you see too many cached images, you should exclude the /sdcard/Android/data folder. To do this, go to Settings, FOLDERS, Manage Exclude folders and exclude this /sdcard/Android/data folder.