DrHorriblejr land

I would assume the image has the green background in the file. You need to edit the image with an image editing software that supports transparency. In which you select the green and delete. A white and faded gray checkered pattern is usually what indicates transparency. Don't know of any free pc softwares but I have this free PaintBrush program on my mac. I usually use macromedia fireworks for pc but I don't believe it's free. I remember photobucket giving transparency editing functionality. You could actually write code to do this, which actually is giving the motivation to write a full program =)