Wish I could help, but I'm compurliterit.(aka the computer reaper) <^\/^>

If you review some of the art that does not display very large, you will notice on the right hand side that the resolution is lower. Revisist how your constructing and saving your files because the images shown on the first screen are a scaled down version of the actual file. If not, I would suggest opening a support ticket with the admins.

A good rule of law for resizing for web is to keep your artwork under a certain size via pix width/height

I make sure ALL my artwork, for web display (unless it's epic and full of detail,) stays like this:
I NEVER let my height be more then 900px
I try and keep my width around 1000 to maybe 1200 if I need.

You need to keep in mind that the common monitor sizes people use is 1024x768 right now, and more and going for 1280x1024 now (like me) And add on the fact that most browsers people use take up the top 8th of the screen, makes viewing tall images harder. Hope that helps ya :3 Keep in mind, just because you use one screen size, doesn't mean everyone else dose too, so if they are to large for you, and you use 1024x768 or smaller, they might be just okay :3