1. Of course, any image you use that you did not create yourself needs to be credited/sourced.
2. Furthermore, just because an image is on the Internet (or anywhere else) does not mean that you have permission to use it. Both ethics and law (copyright) give the creator and/or owner of an image the priority of whether or not to allow the use of an image by others. If you are wanting to use another's image for profit, you must obtain their permission--and also, usually, pay them some fee or percentage. However, under copyright law, the exception for this is for educational use. That is, if you want to use an image for educational purposes rather than for profit, paying a fee is not required and asking permission may not be necessary if permission has already been granted in some way. Two sites which provide such permission (with exceptions) are Artstorand Wikimedia Commons. For links to these, see the box to the right of this one on this page.