Just learned myself!
First, the site does not host images from what I gather though the photo gallery may have different rules so, the image first must be uploaded to another online image database such as flickr.com.
Then you have to get the URL for the photo up to the .jpg extension and copy the URL text. i.e. "http//*********.jpg"
Open a new post and hit the [IMG] button and then paste the URL and hit the [IMG] again and when you submit your post the image should be visible.
Since the image is not on this site I don't think there is a size limit.
Tom.