Free images and pictures
Fre high quality images. All images are available under CC0 license from this source. This means you could download, use, modify and distribute images with no license fees also for commercial purposes. Images from pixabay does not require to state author, but it is kind to credit the author.
Flickr is web space for sharing photos by everyone. Users could provide photos to use them under Creative Commons license. You could choose license which suits your needs and explore many high quality photos.
MorgueFile offers free high resolution photos. You are free to adapt the work (remix), use images for commercial purposes and without attributing the original author. You are not allowed to sell, license, rent or distrubute original images without alteration and also you may not claim ownership of the original images.
Wikimedia Commons owns almost none of the content on Wikimedia sites. The content is owned by creators who added the work to Wikimedia database. Every image contains license (open content or public domain) under which the image could be reused. However, in most cases you do not need to obtain a statement of permission by the owner unless you wish to use the work under different terms than the license states. When you view details by clicking the image in search list there is stated the license of the work.
Openclipart offers database full of clipart. The work is under Public license (CC0) and can be used for both commercial and non-commercial use. Cliparts are available in several formats with transparency - PNG, WMF. Images could be scaled and edited using vector formats - SVG and PDF.