When including graphics and photos in your presentation or assignments, please provide the source. If you created the graphic yourself then "Source: Your Name" is sufficient. When using those from other sources it should include the license (you are checking the usage rights... aren't you?). This could look like "Source: www.some_website.com (Creative Commons license)".
The best place for source statements is immediately below the graphic.
See the image below for an example.
Aerial Photography of Cloudy Mountain
(Creative Commons Zero license)