Darren, are you attaching it directly to the site (via the posting buttons)? If so, they always come out as a thumbnail. Try using an external image hosting site. Which leads me to...
Matt, I use Photobucket. I generally trim my pics to around 800*600 or so for the sake of bandwidth and the size of people's screens, who may be on older laptops. However, Photobucket does seem to shrink them - or present a shrunken version at least - whatever you do.
If this is about the calendar pics, I would recommend just putting "click for full/original size" at the bottom of the post.