What do you do if you want a new icon for an app but are away from your computer? Here's what I do:
Open Safari and do a google image search for whatever subject the icon is on. Find an image, click it full size, zoom in or out so it fills about 80% of the screen and create a webclip. The icon for that webclip will soon become the icon we want.
In the webclip preview, check that the icon is centered. Cancel, re-center and re-create if necessary. Once the webclip is created, go to /var/mobile/Library/Webclips and find the folder for it and copy the icon to the application directory where you want to use it. Now delete the webclip.
You've just created a new icon using nothing but the iPhone.
iPhone and Linux