(Via my reply on the other thread, same info. :) )
Another great alternative, and one I use a LOT for reference images and stuff, is a program called PureRef. Tons of options (opacity, hotkeys, multiple images with ability to align and resize automatically, sort, etc.) I use it quite often for my 3D modeling references, but you can absolutely use it for stuff like this. I have my opacity level adjustments set to Ctrl + and Ctrl - , and it works perfectly. You can lock the window, change it to grayscale, change opacity, etc. etc. etc. Tons of great options, lightweight, and very sleek look (no borders, etc.)
https://www.pureref.com/ if you wanna check it out! :)
Check out my Forza Horizon 3 Gallery
here!