You can change the application icons by simply replacing the icon.png file in the application's folder. Unfortunately, the application can be hard to locate because they are in strangely-named paths like this one:
/var/mobile/Applications/0D93CEFE-F762-4943-A198-17892FEFB7BA/SolFree.app/icon.png
There are a couple of easier ways to locate the particular folder you need. iFile will display the app name above the long hex string, making it easier to browse to the correct folder. If you prefer SSH, you can first use an app called AppLinks from Cydia, which will create links to your apps in the folder /var/mobile/AppLinks.