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. Also, you don't need to actually hit Post Reply to test if it works. Tap More Options and then tap Preview.
I hope that's a brontosaurus.. I might be blind. You're looking as cute as always! Hope you had an amazing Christmas!
If you click the “more options” button and then “preview” you can see if the pic will post or not, no need for test images.