In class you may have discussed briefly some copyright issues pertaining to interesting pages and great visuals. The following links point to relevant information about copyright and the Internet. Be sure to mention that you are a teacher and will not be using the resource for profit.
Many organizations are happy to allow educators to use their resources.
As a general rule, ask before you use it and give credit if you do. Take a look at the references and the information provided regarding this issue. You do not need to read all of these links. Feel free to browse until you have satisfied your need for information.