I know how to use
about:config to block showing images in Firefox; but, for some websites, I need images to be shown; what Firefox extension will allow me to make these exceptions?
I recommend you to use uBlock Origin (Firefox Add-on). You can reenable images in the Firefox config again, if you choose this solution.
Column 1 is for global rules, column 2 for local (domain specific) ones. Forbid loading of images globally (yellow circle). Then you can reenable the loading of images per domain in the column 2.