The safest answer is that if you pay the copyright holder, you will have a license and will not be subject to being sued by the copyright owner. The bloggers could be at risk of being sued. However, there is a a great deal of public domain material on the internet that you can use free of charge. You can also buy CDs of artwork that includes a license to use the pictures on websites.
Be careful doing this. Indeed, there are many many public domain images that you could use on your site. Just find some of those, but make absolutely certain they are public domain images.
But please don't rely on the notion that you can get away with it because some bloggers do. Take a look at these articles before you fall into that trap:
A client of mine was recently dinged by Getty Images for $600 for an image that his web designer casually used as a test piece and never released to the public. The client never knew about it; the web designer didn't think it mattered if it was not posted on the public web. The image came from a source that had snipped the 'Getty Images' proprietary watermark off the photo. None of that made any difference. Getty is not the only vigilant monitor out there (and they use wicked hi-tech image search software to catch hapless users and make them pay) and the law is very strict. I told my client the $600 was a cheap lesson in image usage on the web (a law suit could have cost him thousands).
The problem for honest users like you is that the law applies (with some teeth blunted perhaps) even if the image you select has no copyright notice on it at all. You cannot safely surf the net and find something you like and paste it onto your site. As my client has learned. The other advice to seek a license (often very reasonable in advance) for a particular image you like, or to buy a licensed diskette or download, is almost certainly the way to go.
License the photos. There are a number of licensing web sites for stock photos or specific images (istockphoto.com, getty images). Or take your own. What you are contemplating is willful copyright infringement. You may never get caught, but if you do it will be expensive.
In addition to being a lawyer, I am also a filmmaker and photographer. Check out stock footage sites like www.revostock.com or Getty Images. Both of these companies, and those like them, sell VERY high quality royalty free flat-fee photos that you can buy for $5, $10, or $20 bucks usually, and you then have the right to use them however you wish.
Can you use photos that you don't pay for on your site? Sure. But you are taking a risk if the copyright holder finds out---and then you are liable to pay damages. Don't take the chance--get the rights to post the photos.
Also: just ask! People are more generous with their intellectual property than you might first imagine. If you give them credit as the photographer (if they took the photo), then it's a win-win---you get to use the photo, and they get publicity for their work.
Hope this helps,
Law Firm of Monroe Mann