After almost 4 hours of not knowing, I searched and searched and made this slider with codepen, then hacked away to make it dynamic.
I didn’t write the code, just edited it a little to change the “before” and “after” labels and added a function that allows multiple embeds on the same page. Also works on mobile.
Don’t forget the images need to be the same size. Use the links on the page to see other css tweaks you can do.