Creative Commons - a nonprofit organization that works "to increase the amount of creativity (cultural, educational, and scientific content) in “the commons” — the body of work that is available to the public for free and legal sharing, use, repurposing, and remixing."
Creative Commons licenses provide a way for people to share their work and make it available for others to build on and reuse. It is free to download, adapt, distribute, and transmit without having to ask permission. Depending on the license, however, there may be certain conditions: you may only be able to use the content for educational purposes, you may have to give attribution, etc. Because licenses vary, always be sure to check the exact terms of the license before using an image.
Attribution: others can use the work however they like, so long as they give credit
No Derivative Work: other can copy, display, or perform your work, but it must be verbatim
Non-Commercial: other can use your work, but for non-commercial purposes only
Share Alike: others can distribute derivative works, but only under the same terms as the original license
One option when finding images to use in ads is to look for creative commons or public domain images. The following sources are great places to start. Even some creative commons images require an attribution message, so make sure you look for that!