Both FF and Chrome refuse to open the second one. IE does something weird. Both Opera and Safari figure out the size correctly, but don't display the image.
With Firefox (15.0.1) I get some really, really strange results with the second image.
When I opened it the first time, or everytime I press Ctrl+Shift+R it works, but it shows the URL, and a litte icon in the upper left corner: http://i.imgur.com/B7jFE.png
If I press F5 or Ctrl+R it doesnt work, just as you said.
15,000 x 15,000: http://i.imgur.com/WzCyE.png
50,000 x 50,000: http://i.imgur.com/kgmHu.png
Both FF and Chrome refuse to open the second one. IE does something weird. Both Opera and Safari figure out the size correctly, but don't display the image.