The AMA Manual of Style (11th ed.) does not specify how to cite images used in presentations, so the general recommendation is to cite images as you would cite a website (3.15.3).
Author(s). Title of the specific item cited (or, if unavailable, give the name of the organization responsible for the site). Name of the website. Date published. Updated date. Accessed date. URL
Example
There is no uniform guidance on how to incorporate citations into PowerPoint presentations.
Given that PowerPoint presentations can easily be disassembled, a good practice would be to cite images and other sources on individual slides. You can adjust the font size or put citations in the notes field of the slide.