I find it easiest to have two tabs open:
1. The forum page with the post I am making open
2. My media page where I have uploaded the pictures.
On the 2nd tab tap on the NAME of the picture you want to copy. Scroll down until you find a line called <BBC embed code> long press on the code [smg etc] (and, if necessary, extend the blue select arrows to include the whole of the code) then tap on <Copy>
Go back to the tab with your post and long press on a blank area of the post then tap <Paste>
You should now see something like [smg id=nnnn] (where nnnn is a number).
Complete the rest of your post and then tap <Post>
