I use a python script to create my AOPs now.  The flow is, I import all the targeted images into a single several layer XCF (GIMP) file, resize the images to fit the criteria for AOPs (300x300 or 600x600 depending on type of AOP) and then use a sprite tape (the python script that I mentioned earlier; GIMP sees these as plugins by the way) to create a 10X10 grid.  Create the mask simply by exporting the layer's alpha channel to complete the requirements.