It’s not hard to find images to use online – just use an image search. This works well, but it’s worth noting that doing so isn’t necessarily legal. Most of the images you can find this way are owned by their original creators. This likely doesn’t matter if you’re only using something for private use, or even a school assignment, but if you intend to publish a work you need to make sure all rights are cleared.
Bing Image Search uses a copyright filter based on the Creative Commons licensing system.  The results that are returned are images that have been tagged with Creative Commons licenses.  A link to the source of the image is provided, which you should use to review the source of the image and the applicable license to determine whether your use will comply with the license. (The settings can be switched to show all web results to view more images.)  However, you are responsible for respecting others’ rights, including copyright. Learn more here.
This is TERRIBLE! Besides being able to edit vector images like Will mentioned (which is important for those of us who don't have graphic artists available to create custom images or the budget or time to buy that service ad hoc), you could also search for style "families" (different graphics that were all in the same style). There was a huge range of files and styles in the collection and it was much faster to find what you need than to do a Bing search. Plus, you knew that everything there was free and free to use, without having to worry about CC license issues.