It's important not only to cite the images you use in your powerpoint, but also to use images that have a creative commons license. A creative commons license means the creator has specified that it's okay to reproduce their images. Listed below are some places to find images with a creative commons license. If using Google to search for images, make sure to click on "Tools" and choose one of the options that's labeled for reuse.
The guide linked below includes information about citing images. When citing on a powerpoint slide, use the creative commons license information given with the image. Most freely available clipart does not need to be cited.