Hi,
I have gone through your requirements. You need an application that could get selected google maps satellite imagery. The imagery need to be saved at the highest zoom level, that is maximum available resolution. You require to show only the polygon area specified in the GeoJson file. Rest of the area need to be in black. I can deliver you the application according to your requirements. Further, things like output file name can be automated, if required. For one GeoJson file, one output image file will be saved.
I have more than 15 years of GIS development experience and have mostly working on long term projects. Please look at my portfolio for a few examples of my work.
Hoping to hear from you soon.
Kind regards.