An unfortunate disadvantage with the Web application at
www.printmypack.com is that it requires a previously sized image, and accepts only the jpg format. It seems to have no graphics editing capability on its own.
The png formatted logo linked above is capable of transparency, which is why the FTT logo at the top of this page is not surrounded by a rectangle of white, but instead overlays the wood panel background.
The solution is to utilize your own graphics editing software. With most such applications, you can overlay a png formatted image on top of a background image, then size it for the pack printout app. This can then converted (flattened) to a single jpg image.
Bob