When you are in post reply dialog screen, underneath the 'B' icon box, there's a icon that when you hover over it it says 'Insert Image', click that.
This is what will show up in the typing box those two image tags:
[img][/img]
Now, go to wherever you have an image stored, whatever site you have it (them) posted at and get the image url, I don't know how that works with phone type devices, but I right click it with my mouse (on my laptop) and select 'copy image url' then simply just paste image (ctrl + v) in between the image tags and there it is, visible for all to see.
Example picture below (first the code, then the image as it appears):
[img]https://lh5.googleusercontent.com/-_oXocqOahFw/U-1pJe9oJTI/AAAAAAAAWEU/WkvlgqKNXJY/w1040-h475-no/2014-8-14%2Bhellhound.png[/img]