It should be transparent i guess it messed up and How exactly do you make a Add-on file Sorry im not to smart :/
first make the print, use a preset to make sure its the right size, then, copy the file, and make a folder, one called icon, and another called prints, MAKE SURE THEY ARE THE SAME NAME, then drag your mouse over them, and press the right mouse button, and then got to send to... and select compressed folder, and you name it to:
Print_(type in its spud size, like 2x2r, or the size of the print brick)_(name here)
hope it helps