Where does Android store the snapshots used on the recent apps?
If what are you asking for is the location of the thumbnails displayed by Android when switching apps, then (on my LineageOS 13) they are stored inside /data/system/recent_images as PNG files called x_task_thumbnail.png (x being a possibly random number).

Since the thumbnails are owned by user system and group system, and their octal permissions are 600, you do need root to access them in whatsoever way.