When designing for web or even other things that have to use images, we as designers have many resources. All of these resources have their positives and negatives.
My favorite way to obtain media to use is by our own creation. Photography is a sure fire way to get what you want and know that you are not using another person’s work illegally. A downfall of your own photography is that you might not have the skills or resources that you need to create the piece that you want. The same goes for photoshop paintings or gifs, vector graphics or hand-drawn pieces.
Another way to get stock photos and illustrations. If you go this route, it is usually pretty easy to find what you are looking for. A downfall is that you might not be the only one using an image and that image could have an adverse effect on your branding if it is well-known for something else. Another disadvantage is they tend to cost you some money.
Within the stock images and illustrations world there are rights-managed and royalty-free. These are different when it comes to purchasing the use of an image or piece of artwork. Rights-managed means that that piece is owned and the person who holds the copyright, is in control of who uses that image. They decide how long someone uses it, for what things that it can be used for and how much it costs. Rights-managed can be used exclusively if you get that arranged but they already tend to be pricey, (sometimes thousands), even without the exclusive rights. Royalty-free is when an image is a cheaper option. It required a one time fee that allows for unlimited use of an image, but you have no control over who else uses the image.
One more option is finding free photos and artwork out under a Creative Commons license. This images are free but sometimes have limits. Some you can use freely however you want, others ask that you give credit to the artist, and others are limited to non-commercial use.
Within the libraries of images and creations on the internet, you can find: photos, icons, illustrations, clip art, gifs, and more.
Here are a few links to some sources: