Images shrinking

Here is what’s happening:
I’m trying to display a 900×728 pixel image and it keeps automatically shrinking.
Step 1: I load the image, full size.
Step 2: I insert the image into the post and triple check the dimensions are correct, both in the image settings and the html code.
Step 3: I post the image.
Step 4: The image will display correctly one time.
Step 5: Once the page is reloaded for any reason the image has been shrunk.
Step 6: If I tweak the image or post somehow, sometimes the image will display full size once more, but again will shrink after reloading page.

The site is http://toetagparade.com
Thanks.