I'm noticing more and more issues with box art images lately.
My app always takes the super_url version of a game's box art and then creates its own thumbnails from it, but I see a lot of broken super_url images.
For instance, even on Giant Bomb's page for Freedom Wars (http://www.giantbomb.com/freedom-wars/3030-42671/) the main image for the page is broken, which is actually the small_url image, but the super_url is broken for that game too (http://static.giantbomb.com/uploads/scale_large/9/97089/2621641-untitled.png)
Another example is Mind Zero. Both the super_url (http://static.giantbomb.com/uploads/scale_large/8/87790/2642254-box_mind0.jpg) and screen_url (http://static.giantbomb.com/uploads/screen_medium/8/87790/2642254-box_mind0.jpg) image are broken. More of them might be broken, I just didn't check them all.
I don't have a lot more information for you other than I see broken box art on my app a lot because the super_url version of the box art is broken on Giant Bomb.