Online stock libraries are a great resource for graphic designers. They open up a world of possibilities and expand the potential of a design. You can purchase stock images under a variety of licenses depending on how you plan to use them. Royalty-free stock images are the most accessible, although even though you can purchase them for your use, they are still available for purchase by other designers.
As a graphic designer, I've used stock images for myriad projects. They're very useful when you have a tight deadline and need a photo to illustrate some editorial, to use as 'filler' in a publication, to create a visually appealing cover for a magazine, to add texture to a background, or to use as website content. The images can be used as the main image in a design, or as supporting images.
Stock libraries are usually photography-based, but often include other forms of images: video, animations, vector illustrations and rasterized jpgs.
The wedding invitation collection that I've developed uses some royalty-free stock vectors that I've purchased for use from an online stock library. Each design started as a sketch concept with a shortlist of colour sets, fonts and images. Once I'd refined the design and selected the best image, I've then purchased the vector and modified it to fit the design.
I wish now that I'd considered stock vectors for my own wedding invites. It took far too long to develop the pattern and the swirl used in the design - time that I could've spent working on other aspects of the wedding planning if I'd used a stock library.
The drawback to using royalty-free vector images is that they're still available for other designers to use. Usually this isn't a problem - designers cover a wide variety of products and mediums. It's rare to see a stock image being used in two completely different products, say, a promotional DVD and a local magazine. It's even more rare to see an image being used in two similar products. However, I was recently informed that I've used a stock vector for one of my wedding designs that was also used by another letterpress printer in one of their wedding designs. The image itself isn't even a typical 'wedding' image, so this was incredibly surprising to me. Although we'd both used the image in different ways (i.e. two original designs using the same resource), I've decided not to print that particular design. Unfortunate, because it was one of my favourites.
The moral of this story is do your homework when you're creating a new design using stock images. Even though both printers had purchased the image and were entitled to use it, it doesn't look good to their clients if it's picked up. So that's something to keep in mind.