Just to add a bit more data. I just signed up, as I always liked planetary astronomy. However, like the OP, everything loads except the image. GNU/Linux Slackware 13.37 x86_64 Firefox 12. I have the same issue on MS Windows XP pro 32-bit, SP3, firefox 12.
Tranan, recommending Chrome for this issue is spurious logic. Yes, you may be a "fan" of chrome, but it does not follow that chrome is superior to firefox. Also, from what I can see, the image is loaded within a canvas element and Firefox has supported that since FF 1.5 . See
https://developer.mozilla.org/en/HTML/Canvas
I do not have issues with other sites that use the canvas element to deliver WebGL and graphics.