With the "Send Image" Action, you simply drag and drop it, then paste your image URL.
This is how you can create an "Image" Action:
By default, the image is resized proportionally to fit 80% of the webchat's width.
Multi-platform capability (channels integration)
The image message can be used for all platforms. Most platforms will preserve "animated GIF" as animated.
Ensure that you use an image URL that will be consistent. (Giphy is a handy platform for finding and linking images.)
Google Home (Google Assistant)
For voice-activated devices (Google Home), the image will be ignored.