Below the box you type your message in are 4 yellow / gold bars. The 3rd bar says "upload photos". Click on it and a pop-up should appear at the bottom of your screen. Select the location of your pictures.
In my case I click on "files", then another pop-up appears that has locations of photos. In my case I click on "photos" and my photo gallery appears. Click on the photo you want to upload.
Once it uploads you can click on the "full image" button below the 4 yellow /gold boxes to see your uploaded image full size in the box. That way you can see you got the right photo.
Hope this helps!
