The ideal dimensions are 1,200 pixels wide and 628 pixels tall. Facebook will crop and resize the image preview to 500 pixels wide and 261 pixels tall (aspect ratio of 1.91:1). As Facebook seems to crop the image from the bottom up, try to keep the important details at the top of the image if you can’t keep to the ideal size. Read Buffer’s info on sizing
Our recommended image size is: 640px by 470px to ensure that the most important information is displayed and especially at the top. Images can be longer than 470 px but as mentioned above take into consideration how Facebook resizes images.
Images for emailers would vary. The standard width for an image inserted inside the Newsflash
The standard width for images in emailers and newsletters is 600 px in width. This is to ensure rendering on all devices and this would work for most article sites. Length is not so critical but would recommend no more than 600 px in height.