How can I fix pixelated app icons iPhone 3G

I have just redownloaded several of the Sims games for my iPhone (4S), but it seems that the icons for the Sims 3 Ambitions and World Adventures become pixelated as soon as the game installs (it's normal retina quality when downloading/installing). The game itself is fine, but what can I do to fix this issue?

(clicking on this image makes the pixelation problem clearer)

The middle two icons are not how they appear in the app store, once they have installed.

I have also noticed some pixelation of game elements in several different redownloaded apps, which worked perfectly (at retina quality) before.

Jack