If they are really important then why not pay the less than $1 charged by the likes of iStockPhotos and then you know there are no licensing issues? Only those images on flickr marked as copyright free can be used, others are copyrighted as normal
Also, be careful when you find sites that supposedly offer "free" pictures posted by users. If you use a photo that a user simply reposted from somewhere else in violation of the creator's copyright, you are violating the copyright even though you mistakenly believed that the person who reposted it was the creator. Better get photos from reputable sources who stand behind the origination of the images.
Legally you cannot use a photo you find on the web with out the photographer's permission. Stock photography websites contain thousands of existing photographs that can be licensed for specific uses. A customer who uses stock photography instead of hiring a photographer can save time, effort and money. Typically publishers can either purchase exclusive rights to a single image or they can purchase a subscription of sorts. The subscriptions allow publishers to download a limited number of photos over a specified period of time. Stock photo websites allow webmasters, marketers and publishers to locate pictures for their marketing and promotional materials without the hassle of organizing a photo shoot. With copyright laws businesses must be very careful in using unlicensed photos. There are also risks to using "free" photographs. Many of the free websites contain collaborative works from multiple photographers and artists. While the websites attempt to monitor the images in their collections in violation of copyright laws, there are no guarantees. If you opt to use photographs or images from a free portal, it is important to keep this in mind. My favorite stock photo website is http://www.photo-wizard.net Here is where you can find some other inexpensive stock photo directories: http://www.small-business-software.net/stock-photography.htm Another site list can be found here: http://www.webmaster-elements.com/webmaster-graphics/photos.htm Goodluck
Go here http://compfight.com make a search, then click Creative commons on the left side. It will list only images under the Creative commons license. Make sure you follow the license guidelines (no commercial use + attribution + no derivative works in most cases) Now, if you have any consideration for other people's work, as stated above, spend a dollar to buy a stock photo for your website.