1. If you ssh into your device, go to private/var/mobile/Library/WebClips
There will be a folder for each webclip on your device and inside each folder there is an icon png and an info file.
2. You'll have to go through each folder and find which one is your youtube webclip. (Either by looking at the info file and looking at the url or by downloading the icon and converting it to a regular png with iPhonePNGApp.
Link)
3. Once you find the right folder, replace the icon.png with the one you want and respring.