The images on this site may be copyright protected, may be licensed through Creative Commons, or may be in the public domain.
Before using any images, you need to determine what its usage restrictions are and note that how you intend to use an image can affect whether that usage is permitted under copyright law or Creative Commons licenses.
Keep in mind that anything that is published is automatically copyright protected whether or not it contains the symbol. You may be required to ask for permission and possibly to pay a fee before you can use images for publications or presentations. Images that are "free to copy" often have a note on them that states that they fall under a Creative Commons License or are in the public domain. In the case of Creative Commons, you still need to follow the conditions of the license. Different licenses allow for different types of uses.
Of course, you also should be sure to provide credit for any images you do use to avoid plagiarism.