I have added some covers for the games in the Zork Anthology to the covers website, as well as the Megarace, Castles and Masters Of Orion series :)
What image editing software are you using? Also, why are the covers so large? Looks like you may need to adjust the black levels on your monitor.
The images are scans of old box covers from the 80s and 90s, and the best I have been able to find on the internet. Since they are used copies, the colours are less than perfect. Sometimes the front, back and spine are from different copies of the same game, so there can be colour variations due to this also.