With Creative Commons Tagger plugin youcan easily enable feature for tagging your website images with creative commons license. The plugin simply provides licensing information for media library images and enable you to input Creative Commons licensing information in custom fields in the media post. Finally the license is displayed under the image in wp-caption element.
Creative Common is popular among image files but there are different other content types that are also licensed under creative commons. There are many websites that provides you content like creative common text, creative common videos and creative common music etc. SoundCloud is a popular website you can use for finding some creative common music but on cloud there are no shortage of domains running websites where you can search plenty of it to share, remix and even use it commercially.
Here is a list of some popular websites where you can search for different kinds of images from various categories which you can use with your blog posts as featured thumbnails or content related photographs but remember to cross check the license and always give required credit to images you use in your website.
Openclipart.Org is a popular service that provides you with high quality clip art images and icons which you are free to use in your website or anyhwere, all images are in public domain so you are free to modify or built upon those images by downloading them to your computer.