It's a template. Sadly there is no easy way to save and share custom brushes in Aseprite. You'll want to open up the image file within the program and use the custom brush selection (ctrl-B) to select the brush pattern you want to use directly from the image itself and use that in the image you are creating. I can make a video demo if you need it. I should do that anyway, really-- but I'll do it with haste if requested.