It's kind of a PITA when using a phone. You open the image and in the upper left corner there is a pull down that says, "link", touch that and select "BBCODE" and the link will appear in the box to the right. Here is where it gets tricky, and often takes me a few tries. Touch and hold as close to one end of the code as you can (you need to be touching a word or symbol), two blue dots should appear, drag them so they are at each end of the code and you should have the option to "copy", do so, then hold your finger on the screen in your reply box where you want the image to appear, the option to "paste," should appear, do so. Proceed as usual.
It's more difficult to describe than do once you know how. The hardest part is managing the highlighting of the BBCODE. I did not get the "app" so I don't know if that makes it easier.