There's no standard like in academia, but you should read up on Creative Commons licensing, which is a copyright license that is unique to digital assets (actually, there are a few variants).
If an image is in the public domain, then no credit is required. If it is licensed under one of the Creative Commons licenses, then that site will explain under which circumstances you may use the image (e.g. personal, not-for-profit or commercial), whether you need to cite the original owner and what information to include, and to what degree you can make changes to the image. If there's no license information provided, or if it is marked as copyright (all rights reserved) then you should find an alternative image or create your own.
As for the actual citation, I usually list a URL in the image caption, in small letters. It might not be practical for audience members to visit that URL in the short time that it's on screen, but you may decide to upload your slides after the fact, and then it becomes useful.
An alternative is to list all image credits on a "credits slide" at the end of the presentation. I do that sometimes too, as it makes for less clutter on the image slide.
Hope that helps.