Go to any file, folder, or app on your mac and single click on it.
Single click on the icon at the top of the info screen and press Apple-c to copy it.
Go to your
automator app you made.
Single click on the icon.
When the info screen comes up single click on the icon on the top left.
Apple-v to paste. Your app has a new icon.
Note that when you save your automator app again, the old icon will come back and you'll have to do the process again.
If you'd like to see a few of my automator apps, they're not quite ready for public consumption yet, but they're up, each with their on icon. You'll find them at http://www.booksthatdontgobad.com/DownloadXtras
I don't use any icon builders, but have a few freeware utilities that are nice. I found them either at macupdate.com of apple's software site.
Cocoathumbx-Converts a file's content to an icon. Works with images and pdfs
Thumbscrew-Creates a nice little slanted icon of images dropped on it. Many things can be modified in prefs.