Do an Advanced Image Search, and where it says "Any size" click the down arrow and select "Large"
As teachers, we are protected by Fair Use, but it's worth knowing that you can also change the "not filtered by license" criteria to "labeled for reuse" and you can show your students this.
Do a search for your image - you can select images in "Everyone's uploads" or if you want to teach your students, you can use images under "The Commons"
Grab the "original" picture size and drag to a folder on your desktop in order to use it
If you make a Prezi and want a YouTube video in it, you have to stream it from the network; you cannot first download to a file on your computer/desktop.
加油！
Obtaining Quality Images
Presentation Resources
Principles of Effective Presentations by Oscar Retterer
How NOT To Use PowerpointBy Comedian Don McMillan
Prezi
David Byrne