Once you install it, find the image you want, and save it to a folder in default_db.media (eg, if I want to occlude an animal cell, I would save it to a folder call animal_cell)
I usually call the original file "original.png".
Run greenshot, and open it from the Windows tray - and click on the "Open image from file" option.
Then - click the obfuscate option, or click the button on the left hand side.
Occlude the part that you want.
Use Ctrl+Shift+S to save the image, save it in the same folder as the original file. Name it appropriately (I usually name it what I just occluded).
I then go into Mnemosyne, and create a card using the basic (front and back) type.
Go to the final field and use Ctrl+I to pick the original image. Then, copy the html tag (this is what shows up when you click original.png).
Open one image with an occlusion in the front side, copy the html tag into the back.
Repeat for all the images you generated.