I wrote a very lightweight utility (windows .exe) that takes any image as input (png, bmp, gif, jpg, etc.) - or a folder full of images - and saves the array(s) for use with drawPlusMask to a header file.
Images do not need to have a width multiple of 8, it can be anything, my program will automatically add the padding. Images need to have black pixels, white pixels, and pixels in a third color (any color will do: can be transparent for pngs, but can also be pink, green whatever). This third color is used as the transparent mask data.
I checked the output was fine, but I did not test loading resulting array on an actual Arduboy, so actual tests and feedback would be very welcome.
Utility can be downloaded here: http://mougino.free.fr/zip/drawPlusMask_converter.zip
Just unzip it anywhere on your computer and drag and drop an image or a folder of images on it.