This is always an issue and the bottom line is what are you going to do with the face you find? Put it into a calendar for private use, use it to practice? Usually if your use is only private, it's OK but I would recommend storing the info somewhere, even if it is just the website, which is available to you when you use google. You never know when you may want to go into your archives and use these images at a later date and in a manner in which it would be appropriate to figure out if you need a license to use it, or are asked to otherwise attribute the image.
If you look around the forum, you'll see avatars, signatures, fooling around (such as on Avatar Wars) and these aren't creditied. But they aren't for profit, they're usually really small and the images are non-reproducable. We just don't push the issue in those cases.
There may be a question even if you find it on a government or commercial site like a department store site, but that may be less of an issue. However, if you want to work with high-resolution images (I strongly recommend that you do), there are many websites where you will find quality images, in great supply, with few strings attached. Sometimes all the artist/photographer wants is credit and/or a shared link on their sites, such as most deviantarts.com posters usually ask.
Search here on the forum and you will find numerous threads under the topic, some very recent. This subject never goes away.