I personally use a lot of images on pages and blog posts. Here's a list of some image search engines and databases.
Flickr - This is the most well-known photo sharing and hosting website. Flickr is owned by Yahoo. It's full of generic and amateur pictures, but if you dig around you'll find many professional images.
Some of the images are copyright protected, and you can't save them with "save image as" command. In cases where I like the image too much I'll take a screen shot and then cut it out. This is NOT legal, so watch out.
If you'd rather not run into this problem, there's a way to avoid it. On Flickr, enter your search query in its search box, and then search. After the search comes up, you'll be given the option of clicking on "Advanced Search." Click on that, and scroll down. Near the bottom, you'll be able to click on a box that tells Flickr to search only within Creative Commons-licensed content. This is content for which the copyright holder has chosen to give up certain very specific rights, usually to allow wider distribution of their work.
Google Image Search - Their image search is pretty decent.
Live Image Search - It's sometimes better than Google, sometimes worse.
IStockPhoto - This is the place to buy 100% legal, professional images at cheap prices. For around $10 - $20 you can purchase around 5 - 15 high quality images. Prices depend on image resolution. Before buying you can select the resolution you like, with the smallest being around 400x500 and the largest going past 2500+. You can also buy professional videos and vector images at higher prices. I find their selection to be very broad, and recommend this site.
Stock Xchng - Here you can get professional images for free. I just discovered this site, so I can't really comment how good it is. Try it out for yourself.
Photobucket - This is a free image hosting website. Many eBayers and blog writers use this to host their images. Though it has tons of images, they are usually unlabeled, so it's very hard to find anything. It's perfect for hosting, though.
SlideShare - Slideshare is not related to image search or image hosting in any manner, but since the site is useful I had to include it somewhere. Basically you can share PowerPoint presentations online and embed them on sites, like YouTube videos. Very useful.
Search Optimization 
