It appears that some of the main graphic images used in Apple’s iTunes Store are of a lower visual quality than in the recent past. Look for some of the classic representations of image compression—noise, loss of edge clarity and tone roughness. Compression artifacts are especially recognizable in small and medium sized text.
Image quality changes appeared sometime around August 24-27, but this date range is unconfirmed. The alteration in image quality is consistent across multiple configurations including OS X, Windows and iOS on the iPad (iPhone images seem to still be very hi res).
The change in quality is obviously based on the amount of compression being applied. Apple may have chosen to forgo a bit of image clarity for a faster user experience. The change is not drastic, and most users may not even notice the subtle compression artifacts. This might speed up iTunes, but at the loss of a great looking experience for those of us who noticed…
Update: It seems this was simply a 48 hour hiccup, and all is back to normal—i.e. high-quality images back on iTunes.