even if you cant check the box with the transperancy option it will still be in there. that tutorial adds a buncha steps that arent even needed.
when you make your image starting off with a transparent background all you need to do when you are done painting/drawing creating it whatever you want to call it is go to the layers tab and crate an alpha channel...thats it. you dont need to do all that special crap like copy the image and then change the color overlay or blah blah blah. just start with a transparent backround, and create an alpha channel then save.
i can show you my spray if you dont belive me.
when you save make sure to save in 32bits. 8bits per color channel(RGB) plus 8 for the alpha = 32.