The images you find in Haiku Deck are made available under the Creative Commons License by millions of photographers from around the world. These photos belong to the photographers who agree to share them with you under certain conditions. One of those conditions of this license is that when you use Creative Commons photos, you are supposed to give credit to the photographer by naming them.
Haiku Deck automatically embeds this attribution in the online version of each slide. When you download presentations in .pptx or .pdf formats, the app creates a footnote that includes this attribution information. (see image below)
Though this is not something you can modify in the .pdf or .pptx download, it is editable this in the the *editable* .pptx download. After you open the editable .pptx, just click on the footnote to modify it. While the footnote in the editable .pptx file can be deleted, doing so is a violation of the image license. Some authors prefer to copy/paste this information to a different slide or to a separate document that provides attribution for the photos. Learn more about the Creative Commons image licenses in this article.