I just tested it, and it worked for me... when I select the image, it uploads by itself. There's a little progress bar, and when it's done, the image thumbnail appears. Do you have a popup blocker or something that might be interfering?
I am trying from Chrome on an Android phone and don't get the progress bar. I do if i upload an image to a forum post, but not to a sale post. Maybe I'll try a different browser?