I usually scour Google Images and Flickr and use peoples images on my label. But if you want something more legit, use www.istockphoto.com. It's a good, cheap stock photo and clip art site.
yeah for creating the label, photoshop is the best. For getting the art part, www.istockphoto.com. That's where Zymmie (our Alien) came from. Though he was just a normal Alien, I photoshopped him holding the beer to make him an ALEien.
I use a combination of MS Publisher and PDF's. Print on Avery or similar stock. They clean off pretty easily after a 10 or 15 minute soak. Not a real good choice for picnic coolers full of ice . . . : )
For images I usually use my own, MS site, or something similar.