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

Post photo(s) or an animated GIF in a Tweet.

You need this to get it done:

Photos or animated GIFs
File format: .gif/.jpg/.jpeg/.png
File size: Static photos can be up to 5MB; animated GIFs can be up to 5MB on mobile, and up to 15MB on web.
One tweet can contain up to 4 static images, or 1 animated GIF.
Looping GIFs are supported. If your animated GIF doesn’t loop and plays a single time, it will display as a static image.
Tweet Text (optional)
Add text to accompany the shared photo or GIF loop (maximum of 280 characters.)
Requirements missing or found errors? Report

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

Twitter Help Center - How to post photos or GIFs on Twitter
Information about photos and GIFs on Twitter including advanced options such as tagging, protection and who can see what etc.
Link missing here or found errors? Improve How-Tos