colours more vivid on iphoto than desktop

Hello all,
I am new to Mac. I'll try to be short.
I used a photo as my desktop background on my macbookpro. However, when I opened the same photo in iphoto, it looks much sharper and the colors are much more vivid than on my desktop background.
I noticed the same thing with all other photos. They look better in iphoto than in Preview for example.

Could someone explain why this is so?

thanks,
JM

my guess is that iPhoto may recognise embedded colour profiles, whereas preview may not (although you can match and assign profiles in Preview through Tools.) Perhaps try duplicating an image, assign a profile to one, and test it in both preview and iPhoto at the same time.

Perhpas you have your desktop preferences to "Stretch to fill screen"- would definitely make the image look less sharp if it is smaller than the screen size.