Step one put the picture you want to post in your picture library on your computer
step two click on post just like you did to post this ?
step three right here you have a menu bar at the top of this box ...click on the tree icon
then click on libraries then double click on pictures and pick on your picture you want to post.
If it is too large it will not load
(megapictal wise you might need to resize it first with paint)
After it loads in that preview window click the insert tab and then close that windw and your off to the races.
I attached a screen grab and posted it using my ipad, so I know it works. Not sure about iPhone. I'll try that next.
At the bottom of the mobile version of the page there is a link to go to the full site. If you click that, you should have the "Choose File" button.