I find the site's Gallery software a little painful to use, personally. EMS and others have mastered it. There are a few tutorials here and there describing the finer points of using it, but the problem with sizing has to be addressed before they are uploaded. A good rule of thumb is to re-size them to no more than 800 pixels wide by whatever height is proportional. Optionally, use a service like Photobucket (free) or Smugmug (fee). Smugmug is what I use, yeah it costs a bit, but it's excellent. There is a link to my Smugmug gallery in the footer of this message.
Both of those will generate links for multiple size versions, and you simply copy/paste the link they provide and then paste it into the message using that image tool, which simply surrounds the link with these tags:
[img]imagelink[/img]
Sounds fiddlier than it is - use two browser windows or tabs and toggle between them with keyboard shortcuts. If you can skin a CBX engine you can master this in 10 minutes

N.