Details for how to insert images can be found here.
RPoL faqs (rubbcode)
The basic code example is:
<img src="http://beta.rpol.net/images/banner-light-beta.gif">
There's limited options for image insertion and manipulation compared to standard html.
You'll want to have your image uploaded to a third party hosting service, like Imgur and use a direct link to the image. You'll also need it to already be resized as you can't define display height/width.