You can't do it in trip discussions, but you can do it in the forum section.
I think it's harder to do on mobiles and sometimes features don't work with android at all, however you should see when replying a window with a heap of boxes and symbols on top. The greenish box with a house in it (just to the right of the flag) is where you upload a picture.
Then click upload tab at the top.
Then choose file/ send it to server.
Then if the picture is too big you need to resize it. To the left is height and width boxes, change the top one to 400 usually works fine.