Its called using alpha. simple programs like MS paint cant do this, so dont try. GIMP and PS can. You simply need to allow the alpha channel (as a side note, alpha cant be used in jpeg) and use the eraser or the delete button to remove that part of the image and it will become alpha/ see through. In some cases, team colour can get in the way. And when that happens that part cant become see through, but it will take on the team colour. The only way to change this is through a model edit.
~Dentothor