Overlaying/combining screenshots I have sets of MacBook screenshots with essentially one changing element that I want to combine/overlay on top of one another to create ONE image (not combine into one PDF with multiple pages or stitch images together like a panorama).
I've searched and only found solutions using Photoshop and Lightroom Enfuse (don't have either).  

Any suggestions?
Image editing software is the only way to go here. You'll have to determine which you want to use if you want us to be able to help you out how to do it. If it's a usual window, the easy route would be to only screenshot that window (cmd+shift+4, then hit the space bar and click the window). That will be easy to fill in.