Best practices for Open Graph Images
Open Graph images should be 1200px X 630px. They should be .png (best for logos or graphics), .jpg (best for photos or detail-heavy images), or .gif (if you can't use a .png :-) ).
Use a site like compresspng.com or compressjpg.com to make the image as small as possible. Smaller images load faster, which is a good thing!
Links and References
The best site for nerding out is here: