These are spritesheets, curently 1400x1400 pixels(GM supports up to 2048x2048?). I use background_add and then draw_background_part, with variables for the specific frames. Spritesheets are way easier for adding/updating. I used automated script for making GM sprites from blender before that but i still ended up with huge amount of individual assets, not to mention all the separate frames in images folder. I could go with normal backgrounds but graphical mods won't be possible for the players.It's a terrible idea actually!
Internal sprites are automatically packed into texture pages, which fits many sprites onto the same canvas and has the potential to greatly reduce texture swaps. GM also manages memory differently for internal and external sprites, resulting in external sprites using 4x the amount of memory. So the goal should always be to use internal sprites as often as possible.