I know it can be frustrating finding good card art. I too use Google images, but you must also be sure to actually visit the image's website and scan for any licensing acts or trademarks. Personally, I usually append my image search with the words "stock photo." This usually weeds out most of the stuff you can't use anyway (unless you feel like paying $200 + just to be able to borrow the picture). The downside, of course, is that the majority of stock photos have big ugly marks or logos on them (watermarks).
After finding a photo I can use (this usually takes an hour or so) I run it through either Picnik (
http://wwwpicnik.com) or Befunky (
http://www.befunky.com) to alter the picture enough to make it my own. Bear in mind we're still working with stock pictures so it can never truly be "your own" image, unless you alter it to the point the original image isn't even recognizable anymore. Mostly I run the images thru overlays & filters, if only to give the image a CCG "feel." Nobody wants somebody's family vacation picture on a card, they want to see something cool and unique. Sometimes just adding a blue overlay to a picture can make it "artsy" enough to suit a water deck, or try sharpening the details for a grainier effect on Time & Earth cards.
Or, coming from another perspective: I have a friend who is quite the shutterbug, takes lots of pictures. She specifically watermarks all her work so that nobody can steal them. Plus, it's free advertising for her whenever somebody DOES see the photo, so in that regards she usually has no problem letting others use her pics, provided the watermark stays. However, there was another friend, of whom she had taken a picture, who decided to crop the watermark out of the photo & use it on facebook as a profile pic. While the friend meant no harm, this is technically stealing. And believe you me, my friend with the camera was pissed. She puts a large amount of time & effort into her photography & it's how she makes her money.
Overall, just be sure to use something nobody will either charge you for or mind letting you use. Sometimes, all ya gotta do is ask!
PS - there's a user-submitted art section in this very forum