Each social network has its own suggested image size, which creates a lot of difficulties for marketers. For example:
What Sizes You Should Be Using:
1. LinkedIn posts a thumbnail sized image at 180 x 110 pixels
2. Twitter has a 2:1 ratio and suggests a 1024 x 512 pixel image
3. Additionally, the 2:1 ratio for Twitter is not quite accurate on their iOS mobile app; I’ve discovered that images are cropped significantly on the left and right borders and displayed on the iOS mobile app at a 1.75:1 ratio
4. Facebook suggests a 1200 x 630 pixel image
5. Google+ seems to post images at 345 x 195 pixels
After crunching the numbers, I discovered the following image size works for all the above: 1600 pixels wide by 800 pixels high with a 160-pixel padding on the left and right margins, like the example in this post.
More details - Garret Heath, Rackspace.
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