You can change it in iFile. The file path is var/mobile/library/webclips.
The image name is icon.png and the size is 114x114. Delete the existing image and add one that you like. Then open the Info.plist and make sure "IconIsScreenshotBased" is set to false. If it's set to true, it will change back to the original after you open the webapp. Save the Info.plist and respring.
I'm quite fond of customizing my web clips using iFile. Instead of manually editing the .plist using the Text Viewer option, which isn't that difficult, I prefer using the Property List Viewer. When using this method you can simply hit a couple of toggles and it keeps your custom icon. The toggles in the second pic are how they're set for one of my custom web clips.