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

The ability to publish articles is available to all members (currently not available on mobile devices, however.)

Requirements
You need this to get it done:

Headline
The headline for your article (maximum of 150 characters.)
Content
The content of your article (maximum length of approximately 124,000 characters.) Basic text formatting is available, including hyperlinks.
Cover Image
The cover image appears at the top of your article, and also acts as a thumbnail for your article throughout LinkedIn.
File format: .tiff/.gif/.svg/.bmp/.png/.jpg/.webp/.ico
Recommended resolution: 744 x 400 pixels
Cover Image Credit/Caption
There is a small text field underneath the cover image where you can credit the photographer/artist/etc of the image and/or add a caption (maximum of 250 characters.)
Multimedia (Image)
You can add images to your article.
File format: .jpg/.png/.gif (static)
Maximum file size: 10MB
You can also browse Flickr's Creative Commons inventory of photos.
Ensure that you have the rights to use the image(s) you wish to include in your article, and credit/reference any sources as necessary.
Multimedia (Video Embed)
You can embed videos from video sharing platforms such as YouTube, SlideShare, TED, Getty, Vimeo, Vine and Lifestream.
Make sure that you're including the URL (https://...) and not the embed code (< blockquote class="instagram-media"...). The URL can be found in the navigation bar at the top of your browser.
Multimedia (Slides Embed)
You can embed slides from slide sharing platforms such as SlideShare, Prezi and inVision.
Make sure that you're including the URL (https://...) and not the embed code (< blockquote class="instagram-media"...). The URL can be found in the navigation bar at the top of your browser.
Multimedia (Rich Media Content Embed)
You can embed rich media content into your article, such as tweets and Instagram images.
To embed a tweet, click the "... More" icon at the bottom of the tweet card. and then click "Copy link to Tweet" from the menu.
To embed an Instagram photo, open the individual photo and copy the URL in the navigation bar.
Make sure that you're including the URL (https://...) and not the embed code (< blockquote class="instagram-media"...). The URL can be found in the navigation bar at the top of your browser.
Note that you can’t embed other LinkedIn Articles in your article on LinkedIn due to privacy restrictions.
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:

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Publish Articles on LinkedIn | LinkedIn Help
Information and requirements for writing and publishing an article.
Official
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Add Images and Other Rich Media to Your Article | LinkedIn Help
How to add images, videos, slides and rich media content to articles.
Official
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Tips for Writing Articles on LinkedIn | LinkedIn Help
Useful tips and inspiration for writing articles.
Official
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Official
Link missing here or found errors? Improve How-Tos