Not all that many years ago, while working as editor of a community newspaper, when a graphic element was required to dress up editorial content or design an ad, I ran to the good old cut-and-paste book then leafed through page after page, image after image to find just the right one. The result was that many times I settled for an okay illustration because deadline was looming.
A digital catalogue of the images eventually went online a few years before my departure, enough time for me to know what a difference it made — in time and in creativity.
However, while the range of images is incredible, there are times when the terms state a little alteration is necessary, even though you love the illustration you have. For example, some sites, such as iCLIPART.com ask that the images be edited for commercial purposes. Changing or removing a background will help to meet that requirement while leaving the primary subject as is. You can learn how and see some examples by checking out these sites:
Digital Arts Lab Quick Background Change
Exposure Guide Change the Background of a Photo
tipsquirrel Changing the Photo Background
Media Militia Removing a White Background
Graphics.com Mastering Photoshop's Background Eraser Tool
wikiHow Change the Background Colour in Illustrator
Digital Trends Two Foolproof Ways to Remove a Background