It looks like you just copied the gallery link and then manually added ".jpg" to the end of it. You need to press and hold on the image, and then Copy Image URL. The link should then already have .jpg or .png at the end of it, not you add it yourself.Sorry I was testing, how do you get it to post like that?
Also, you don't need to actually hit Post Reply to test if it works. Tap More Options and then tap Preview.

