To be extra careful, and in risk of not being able to use the images, you can contact the webmasters before hand and request their premission.
Otherwise, you can merely put a notice at the bottom of either the picture or the thread (should you be posting one).
What I find convienent is a 'Credits' page, if you're designing a website. Use the pictures freely, put something small and not incredibly distracting (like [size=7]1[/size]) next to the image, and then on your "Credits" page, go through and give credit. For example:
Image 1:
http://www.blahblahblah.com
You seee? But that's me, and I have too much time on my hands.
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