This is the bitmap containing all the sprites. It's decoded from a personal compression format then send to the video card. All sprites rendered in the game are actually a quad textured with a part of this bitmap.
This file is stored in a 24bit RGB format (8bits per channel). When loaded an alpha channel is added (32bits RGBA coded). For the pixels carrying some arbitrary chosen values (eg. :
#00FE00, a really close colour to plain green
#00FF00), the alpha channel is set to 0. In this example, Alex running sprites then don't display the green pixels.