Your application icons are in the Applications folder. Each program has it's own folder, like Nes.app. Finder will launch the program folders as executables, so disable application launch in Finder's settings. This will let you browse their folders to examine their structure.
After you've nosed around and found what you want to replace, upload your file to your sendspace account from your desktop. You created a sendspace account when you registered your ishare account.
http://www.sendspace.com
After you upload browse your files and find the one you uploaded. Click it to find it's download url. Take note of the last 6 characters and enter them into iShare. Download the icon.png file to your desired application folder.
Restart your iphone to see the changes. To restart hold your power button down for a few seconds until you see that red slider message to turn the phone off. A quicker way is to launch the springboard prefs and just tell it to restart springboard.
Come to think of it, you're better off making your own Springboard 2.1 theme. Upload your files to the springboard theme folders. I covered it in another thread in this forum, shouldnt be hard to follow.