Like, the link to the image? If it's your image, you can host it on whatever site you like (I use imgbb, some people use imgur or catbox.moe) and it'll give you a link that you can use. If it's someone else's, you can usually long press the image, select "Open link in new tab" and copy it from there. It's basically the same syntax as posting a link, just without anything in the [ ] brackets, and with the exclamation point in front.