First, you host the photo on some free hosting site, like Google photos, imgur, etc. You can even post it on Facebook, and get the image link from there, of course the photo on those sites would have to be public.

Then you do the image HTML here in the body of your post to link to your photo. For example:

<img src="https://i.imgur.com/llXyOtC.jpeg" />

That will pull your photo in, like this: