Comparing your images to mine, yours are set to a much smaller resolution than mine (72DPI as against 240) I suspect the root of the issue is that your images are just small, which may be down to the settings in your iPhone.
Also I notice that in the 'share image' screen you don't have the option to use BBcode (but that's possibly because I'm not the owner of your account) If you do have the BBcode option, copy the URL from the box in that window, below that box is a 'drop down' menu to select the size. Set the Size as required and paste that URL into the text body of your post.
The T'interweb will sort the pic out.
Try that..