You can find Facebook ad guide here: https://www.facebook.com/business/ads-guide/image/facebook-feed/post-engagement
File type: jpg or png
Normally I use this image ratio: 1.91:1 for landscape images.
So if you have a width of 1200 px. You can divide that by 1.91 to get the height.
Ex. 1200 / 1.91 = 628 (height) or 628 x 1.91 = (width)
I also use the 1:1 ration (square) many times even in videos as it fits well with many of the different platforms like Instagram.