On PC with a browser, click on the Picture icon in the Reply box...
Use the Insert Image popup window to select files from your PC or insert hotlinks to images on the web.
From a phone, I have heard Tapatalk App allows uploads (many who use it say so) and a recent rumor leads me to believe an update to the Windows Central App may now allow for uploading pics (one off comment I read somewhere).
I don't use such apps so I can't give a tutorial.