You have to host the image online and then use the url to paste here. You can host them on Google Photos and hit the share button and it will give you a link
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Go to somewhere like Imgur.com to post the photo and then use the url there. Amazon's photo storage works also.
I imagine that Apple offers something similar.
I prefer Imgur.com because it is so easy. Create a free account, right click on your photo and hit copy, click on your Imgur page and just paste with <CTRL> "V". and there it is!