Don't use the /a/ links from imgur
When you upload a picture, you get two links:
The one with an /a/ and the one without the /a/
/a/ stands for Album, and you're directing bbcode to an album and not an actual picture, so an error happens
You have to use the one without the /a/ to post pictures.
In this example, you would have to use the "http://imgur.com/gixL3Es" one
Once you have the link, you go to the image bbcode button:
And click it.
You paste the link from before in the box (the non-/a/ one):
AND you also have to put ".png" at the end or else you won't get an image
And now, when you click OK, the image appears:
For phone just hold the image for a bit and then tap "open image on a new tab"
Once that's done, copy the link and use it on the bbcode box