no sources

Always reference your sources when adding images, citations or texts that are not initially created by yourself.

You do not have to provide the link to an used image on the slide where it is shown but can also make an additional slide at the end of your presentation where you list all image sources. That page does not have to be shown in your presentation but is good when giving away the file later. Most of the time it does not create any added value for the audience to listen to you while you read all URLs out loud at the end of a otherwise good talk.