Alright so I downloaded Gimp and after playing around with it for a little bit. I
think I know how to do it. First open the image, then to cut the specific sprite you want select the dashed box tool (I don't know what Gimp calls it but in Photoshop it is called the recantgular marquee tool). Afertwards do ctrl-c and open a blank file. Now before you click okay in the dialog after selecting new go to Advaned options and find the drop down menu that says fill background with: select transparent. Now you should have a new window with a checkerd background. Then do ctrl-v and your sected sprite should appear. Next click on the tool that looks like a magic wand. In Gimp I think they call it the fuzzy tool or something. With this tool selected mark the specific sprite part (not the background) and the sprite area should change slightly. Afterwards right click and go to edit>invert. Finally press the delete button, and there you go!