Every LinkedIn image size
Most people need three LinkedIn images: the profile photo, the banner behind it, and the images in their posts.
Post
1200 × 627 px, 400:209 Landscape post image, same shape as a link preview.
Banner
1584 × 396 px, 4:1 Background photo on your profile.
Profile picture
400 × 400 px, 1:1 Shown as a circle, overlapping the banner.
A profile photo and banner that work together
On your profile, the photo sits on top of the banner near its lower left. Whatever is in that corner of the banner is hidden, so put text, a logo or a tagline in the right half.
The banner is 4:1. A landscape photo resized to it loses most of its top and bottom, which is why a wide, simple image (a desk, a skyline, a plain colour with a few words) usually works better than a photo of people.
For the profile photo, a head-and-shoulders crop reads well at the small sizes LinkedIn uses next to comments and messages. Resize from a photo that is already square or close to it, so the center crop keeps your face where you want it.
Images for LinkedIn posts
For posts, 1200 × 627 is a safe landscape size. A chart or a slide often works better as a single clean image than as a screenshot with browser edges around it: crop it to the size here and the feed shows it without extra bars.
If you post on LinkedIn regularly, write and resize a week of posts in one sitting and schedule them, rather than doing it every morning.