Your comments

Thanks for reporting this. It's been fixed.
The large thumbnail is 640px wide. If you're image isn't that big to start out with then it will just show the original image, which is smaller. It won't try to make a thumbnail that's bigger than the original.
Sorry, we can't support RockMelt browser and the site remains untested with it. We support Chrome, Firefox, IE and Safari.
That option is only available from the account section. It's not available from the home screen because of the massive load it takes to resize lots of images at once. So, you need an account.
I'd be glad to help you out with this -- it should definitely be working. What browser (and version) are you using? Have you tried using other browsers, or disabling addons?

Also, make sure you have Adobe Flash installed and working correctly.
I'm not sure what else we can do then. Usually when people have problems with images not loading it has something to do with their DNS.

There aren't any service outages anywhere right now, so I have a feeling it has something to do with your computer/network. Have you tried loading images on any other computers?