You may want to try out the fixup "Mark all non-structure elements as artifact" which you can find under the list of fixups in Acrobat DC Pro Preflight (not yet available in Acrobat Pro XI).
To open the Preflight window, use Edit -> Preflight, or Cmd-Shift-X, or the resp. tool in Print Production/Standards/Optimize PDF.
In the Preflight window you will find three icons next to the "Show all" drop down menu - click the rightmost one of these three icons to get to the Fixups pane.
Use the text filter in the upper right of the Fixup pane to search for 'artifact' - the fixup "Mark all non-structure elements as artifact" will then be listed.
Apply this fixup to your tagged file ***after*** (sic!) you have tagged all actual content.This fixup will then mark everything else artifact (that;s the technical term for "background content").
Just to play it safe, you will want to keep a copy of your file right before you apply this fixup (so do not simply overwrite your original when applyign this fixup).
If the presentation will remain in PowerPoint format - not converted to PDF - select each non-meaningful image and set the Alt text to "" (that is, opening double quote immediately followed by closing double quote). A screen reader will then ignore those images. If the presentation will be converted to PDF, Olaf gives a great suggestion above.
Thank you, Olaf, but the presenter chooses to keep the file format a PPT as it's what he's familiar with (and may want to edit further after accessibility modifications)!
Thank you for the response - yes, I love the artifact feature of Acrobat Pro . . .makes the accessibility work much easier!
C student - thank you . . . I will try this. I was looking for something quick like artifacting images in Acrobat pro so they're just set as bg, and while this method of the double quotes means I still have to open each up and put something in the alt tag pane, rather than selecting multiple images and tagging as bg, it's still going to be quicker than typing in full descriptions of the images. Is the double quote method an official fix in PPT for having a screen reader skip an image or is it a well-known feature of screen readers?
I wanted to add one last comment to this string should anyone in the future be looking for solutions to this issue as I had been (even with 'a C student's comment marked as the Correct Answer . . . I'm unable to verify if this really works as I don't have a Jaws screen reader, so chose the answer as correct based on it sounding logical). I found the following comment on WebAIM: PowerPoint Accessibility:
"There is no way to add empty/null alternative text to an image in PowerPoint. If your presentation has decorative images that do not convey unique content, the best thing to do is to leave the alternative text field blank. If the image does not have alternative text, it will typically be skipped by a screen reader."
. . . and to be thorough for anyone who's reading this in concern to older PP versions on a Mac, I should add their disclaimer:
"All versions of PowerPoint for Mac through 2008 have serious accessibility limitations. For example, you cannot give images appropriate alternative text or export the presentation as an accessible PDF file. Because of these accessibility limitations, this article does not include guidelines for Office for Mac users."
Hope this helps somebody somewhere
[above WebAIM article: Last updated: Apr 18, 2014]