Action Details
Requirements and how-to articles for LinkedIn's action:

You can share photos with your network.

You need this to get it done:

You can select up to 9 images to share.
File format: .gif/.jpg/.png/.bmp
Maximum file size: 5MB
Recommended minimum resolution: 552 x 276 pixels
Aspect ratio: Between 3:1 and 2:3 (if the ratio is larger, the image will be centred and cropped)
Note that uploaded images cannot be resized and small/low-resolution photos may appear low-quality.
The content of your post (maximum of 1,300 characters.)
Posting Location
If you are an admin for a company page(s) you can choose whether to post from your personal profile or from a company page.
You can choose who your post is visible to - Anyone, Connections only or Group members. You can also choose whether to enable or disable comments on your post.
Hashtags (optional)
You can add hashtags to your post in order to help it get discovered by other users. However, be careful not to add too many (the optimum number is generally believed to be between 3 and 5) as it could risk looking like a spam post.
Tag People/Companies (optional)
If other people or companies in your network appear in your photo(s) you can tag them.
Alt-text (optional)
You can add alt-text to images which you share. This allows members using voice-over screen readers to understand what’s in the image (alt-text won’t be visible to members not using screen readers). You can add a brief (maximum of 120 characters) description for each photo. This is optional but recommended.
Requirements missing or found errors? Report

How-To Articles
A list of useful links for you:

Posting and Sharing Content - Overview | LinkedIn Help
How to write a post and share it with fellow LinkedIn users.
Sharing Photos or Videos | LinkedIn Help
Information about sharing photos and videos.
Link missing here or found errors? Improve How-Tos