is it legal?

Yes               

of course! Although copyright laws nowadays are a b****......

I am a photographer, and copyright is a pet peeve of mine.  It's very frustrating to buy all my gear, take time to set up a shot, maybe after traveling to get there, take multiple shots, edit, save upload etc etc and have it appear where it should not because google linked to it and someone thought that meant it was free for the taking.  Believe it or not, some photographs take hours to make, waiting for the sun to be in just the right place, and they can be costly.  I recently had to purchase a 150$ filter to get a photograph just right (in addition to the 4000$ lens that I used).

ALL images are copyrighted, from the very minute the camera owner pushes the shutter release on their camera.  Unless otherwise stated, most fall under the fair use or educational copyright, allowing them to be used for educational or non-profit type stuff (not on personal websites)

That being said, some of the images on Google have a full copyright and cannot be sued for any purpose without consent of the photographer/graphic designer.

If you want to use ANY image and be legal (and moral) please check that individual images copyright.  The photos on google and yahoo are linked images.....you can click to go directly to the website that they are on, (meaning Google and Yahoo DO NOT own or have rights to those images).  

You can link to an image and be ok, in most situations.  But to just use the image without asking, well, that may be illegal.

If any of you want to use a specific image and are unsure about the copyright, PM me and I'll check it out for you.

It does say on the Google images page that pics may be subjected to copyright.

http://www.backyardchickens.com/t/630822/security-for-your-duck-photos#post_8439928