I'm not familiar with Imagevenue, but their FAQ says you cannot hot link images without paying for their "premium" service.
I suggest using Photobucket, or Imageshack, or even Flickr. I have created a tutorial for using Photobucket /forums/654172/ShowPost.aspx which is basically the same for all formats.
What you want to end up seeing in your post before hitting submit is something like this:
[ img ] imagenamehere [ /img ] only without any spaces.
Good luck!
Bill
Edit I've just had a read through the rest of the FAQ on Imagevenue. I'd get off that ASAP, as their service sucks. You can't delete images without sending them an email requesting it? Ridiculous! /edit