A general rule of thumb is for images to be at least 1200px wide. That ensures good quality on all networks.

For specific networks, please consider the following:

• Instagram images can be 1080px wide and can be somewhat vertical (up to 4:5 aspect ratio)
• Facebook can have vertical images when the content does NOT have a link, in which case those can also be 1080px wide
• LinkedIn link shares (with a link) will always crop to horizontal. LinkedIn displays all images at a 1:1.91 aspect ratio, and crop square images in that way - this is how LinkedIn crops images