Question 1

What is 851x315 used for?

Accepted Answer

851×315 is the Facebook cover size, used for Facebook page covers. Matching it exactly avoids the platform cropping or re-compressing your image in a way you did not choose.

Question 2

How do I resize an image to 851x315 without stretching it?

Accepted Answer

Crop to the right shape first, then scale. Scaling a differently-proportioned image straight to 851×315 distorts it; cropping to 2.70:1 first and then scaling keeps everything in proportion.

Resize to 851 × 315