Depends on the use and jurisdiction. Generally, though, no. Using an image from another site for commercial purposes without permission would be a bad idea.
(disclaimer: I am not an attorney)..
Better to be safe, always ask permission if you can't do that, then at least create your own graphics or produce your own photos/pics...
Even with credit the owner would have rights to request you remove their image.
The best thing to do would be contact the copyright owner and request permission to use their image and let them know you are happy to provide a link to their site and credit in exchange for the use. I have happily let people use some of my stories, articles and images this way and have even gained traffic from this but I personally would take offense at it being used without asking and so might they...
It's *always* illegal to use images you do not own the copyright to. The only case where it is not if if the owner of the copyright, such as the author, gives you explicit rights to use it, such as adding releasing their images in the public domain or under a Creative Commons license.
In the case of photos it's even more complex, because if for example you have a photo that was taken by a photographer, you may own the photo but you would not own the copyright to it, the photographer would.
One good source of images is Wikipedia. All the images shown on Wikipedia are required to be reusable, so you can use them if you follow their license, which usually is either in the public domain, or requires credit.
On a related note, if anyone wants 3D images to use as part of backgrounds, there's some I created available in the wallpapers section of http://www.3dfantasies.info which you can use as long as you link back to the site...
Just to make the point the others already made, stick a little more... no it's not legal without permission... so do your homework and ask..