Credit the source of the image, if you are using free images from the web. Do not reproduce images without permission. Some images are labeled as "public use" images but always check the permissions for each image.

Flickr

Google Advanced Image Search allows you so filter results by usage rights 

Openclipart.org: 

When citing sources within a presentation:

• you can include your references as in-text citations on each slide
• or provide a reference list slide at the end of your presentation with corresponding in-text citations
• or combine these and have in-text citations and a reference list.

Make sure your audience knows where you obtained the information, visuals, and other materials you used in the presentation.