To deal with sprite sheets it's better to load the sheet as a bitmap then grab all the images you need off that sheet. Loading it as bitmap numbers 1 to 32 hides it from the user so you can do anything you want to the bitmap without the user seeing anything. When you're done change the working bitmap back to bitmap zero (which is the main screen). To save the alpha channel using GET IMAGE you just add SET BITMAP FORMAT 21 before the first bitmap creation or GET IMAGE command.
` Allow saving the alpha channel for all future GET IMAGEs (21 is 32 bit)
set bitmap format 21
` Load the image as a bitmap (to hide it from the user)
load bitmap "sprite.png",1
` Grab your image (use 1 as the texture flag so the image doesn't blur)
get image 1,0,0,100,100,1
` Change the current working bitmap back to the main screen
set current bitmap 0
` Cleanup bitmap that's no longer needed
delete bitmap 1
` Show the sprite
sprite 1,x,y,1
wait key
