Flickr is one of the biggest sites where you can add your professional or semiprofessional photos, and create a nice showcase. While some photos have some restrictions (no download allowed, no use in commercial projects etc.), other images are free to use in personal projects and as well in commercial projects.
So if you are a talented web designer, theme developer, and so on, you will definitely need some nice images that you can use in your commercial projects.
How can I search for creative commons images?
All you have to do is go to Flickr.com Advanced Search and at the bottom of the page check this two boxes
Then hit Search, and start browsing. You can download and use any image from the results, in commercial projects.
Note: Make sure you read the creative commons license however, to understand what you can do, and what you can’t.