Stock photography and clip art
By Craig Goldwyn, visibility.tv
Stock photography. Hiring a photographer is expensive. But the results are almost always better than shooting it yourself. Don't make your highly professional law firm look amateurish with mug shots taken by a secretary in the conference room. There are numerous libraries that have excellent photos you can buy cheaply. Some are huge libraries with the work of many photographers, and others are the work of a single shooter.
Clip art. As a rule, original art is better than clip-art. Especially the cheesy stick figures that you get from Microsoft. Whenever possible use your own graphics or hire an artist. But occasionally clip art comes in handy for backgrounds, icons for buttons, or logos.
Here are some sources. The list is by no means comprehensive.
http://www.corbis.com - World's largest stock photo seller and buyer.
http://www.gettyimages.com - One of the largest photo sources. Current news photos, too.
http://jupiterinages.com - One of the big three.
http://www.freeimages.com - Clipart, backgrounds, arrows, faux fur, all free.
http://www.cartoonbase.com - A searchable database of cartoons and cartoonists. Some funny stuff here...
http://www.laughing-stock.com - Laughing stock has high quality off-beat illustration.