The only problem about grabbing images off the web is that a copyright to that image is held by someone in most cases, and unless you have permission to use the image, or the maker of image has expressed a copyleft or other grant of use that covers your usage, it is a violation of copyright law to use the image. There are images that are expressly in the public domain, but most images you find on the web are not.
Are you likely to be stopped from using the image for labels for your own personal bottles of beer? Probably not, because almost no one will ever know that you used them. You certainly couldn't go commercial with one. But you should at least consider the implications of using someone else's protected images without permission, given intellectual property law as it exists at present.
Having said all that, I personally don't subscribe to the notion that intellectual property is equivalent to tangible property and can be legitimately subject to the same protections. But my view of IP, sadly, is not the law, hence my comments.
