You could load it in Windows Paint (if you use Windows) and press "Resize". Then make sure it says "Percentage" in the window that pops up, then change the 100 to 50 (it should change both numbers if this is your first time using it). That should scale down you image. However, if their's any parts that are supposed to be transparent, you'll have to use something like GIMP or Photoshop to make it transparent.