If you use Google's Picasa web hosting for pics, here the trick:
Open the picture you want to post. On the right side will be a panel. Click on the small triangle next to the "Link to this Photo" link. It will give you more options. Click to select "Image Only (no link)" And do "select size" as "Large"

Then copy the "Embed image" link contents and paste it into your message, and put [img] and [/img] around it.
[img]paste_what_you_copied_here[/img]