The social media sites are constantly changing the best photo dimensions but we seem to be approaching a general rule of thumb of about a 2:1 aspect ratio.
For Facebook it is a bit larger, Google+ a bit longer and Twitter is in between.
Get More Likes for News Releases
When sharing stories or news release from your website to Facebook make sure your first image or featured image is at least 600px by 315px, but for the best look on iPad’s and other retina display devices your picture or photo should be 1200px by 630px.
Get More Retweets for your Photos
For for twitter stream photos, the preview screen is about 2:1, and for the best result on on devices use a photo at least 1024px by 512px.
Does this look right to you? pic.twitter.com/x176OTkwlt
— Brendan Mulvihill (@MP_Host) November 25, 2013
Quick Cover Guide
Google+ recently updated their cover photos bringing them more into line with dimensions you’d find for Facebook and Twitter.