Removing slide design objects from the slide master for screen readers

I am trying to make my slide background objects hidden to screen readers.  I'm using a background that I chose from the ones available on the design tab, and I'm unable to find a place to modify it.  The tab order screen is showing both a picture and shape on the slide master, but when I view the slide master, I cannot select either one to add alt text or to make it hidden from screen readers.  

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