It depends on the programmer. Some like individual images and some like a grid. Typically if it's for an animated sprite it should be in one file so they can just use the CREATE ANIMATED SPRITE command. Make sure the size of the image isn't an odd number or there will be a wobble when it's played. The attached file is 434x32.
set display mode 640,480,32
sync rate 0
` Create an animated sprite 14 frames across 1 down
create animated sprite 1,"MouseAnimation.png",14,1,1
` Animate the sprite frames 1 to 14
play sprite 1,1,14,100
` Show the sprite at the mouse coordinates