If you reply (rather than the quick reply) there is an option to attach a file. An example of how that looks is below.
You can also go to your gallery and upload things that way.
Outside of that, if you just want to display an image from the web the normal bb img tag works.
Click the little painting looking thingy in the toolbar:

As I already said I have clicked on the little painting thingy. Then What? It does not allow you to browse or paste anything in there. What and how do you get an image in there. Totally clueless. everywhere else on the interweb it allows you to browse and grab an image. Not here.