Never assume an image does not require attribution. If you’re going to include an image, create your own or find images that are labeled for reuse.
You must check the re-use restrictions on each image you find. You can use advanced settings in Google, Flickr, Bing, or Yahoo to filter by license. Or, you can search the sites below. These are my go-to sites when putting together presentations. You’ll notice, I have a tendency toward health related images. Dig in!