I think the original images weren't much higher resolution that 2560x1440. Some of those images are many years old back when cameras were limited more in terms of megapixels etc. I noticed that the background images looked a bit blurry when I got my 30" ACD. They definitely look sharper on my MacBook Pro which has a lower resolution.
i think its more the other way round the native resolution of the pictures is 1280x960 on a 17-20" screen so if you stretch them to this huge 27" they will become a bit less sharp, something thats quiet normal so nothing to worry about ,just go down with the resolution to the original resolution of the picture and it will be sharp again