Basically you need to navigate to the photo in your photobox account and bring the photo you wish to embed up on the screen, full image if that makes sense. Then you copy the URL from the address box in your browser and paste that into the message using the Insert Image icon. You may need to allow scripted windows to do that, so be on the lookout for a pop up or prompt to that effect.
There's an explanation in the FAQ forum of Canreef. Here's a link. The Attachments & Images info is part way down the page & the info you need is at the very end of that box. Not the part about attaching an image, but the part dealing with an image on another website, in your case photobox.
http://www.canreef.com/vbulletin/faq...b3_attachments