To add an image in your Canned Actions, you'd have to use HTML, as dragging and dropping doesn't work. Sounds intimidating, but it's rather simple. Just follow the steps outlined below:
Step 1: Upload the image to a publicly accessible location. Example : Dropbox, Google Drive etc.
Step 2: Go to your Canned Actions page.
Step 3: In the Insert Reply section, click the HTML source editor button.
Step 4: Add the image link inside a <img src=""> tag as below:
Replace your-image-URL-source here with the direct link to your image. With your image URL and the tag, it should look something like this:
Step 5: Click Update and the image will then appear, which you can then resize by dragging the corners.
Once you’ve done all the editing in the Insert Reply field and checked the other options in the Canned Actions, click Save Setting to confirm your changes.