To change a photo that’s locked on the Slide Master, first go to View> Slide Master, and you’ll see all the layout options available to you. These aren’t being seen in the final presentation, but they are the starting place that your presentation bases it’s design on.
To change the image, select the picture and go to Format Picture>Change Picture. Locate the photo you’d like to use, and click Replace.
Because the photo you choose might not be the same dimensions as a Powerpoint slide, you may need to change the size of the photo to make it full screen.
From Format Picture, click crop, and hover over the edges to get your cursor to change to a black T or L.
Expand this to the size of the slide, and then enlarge the photo to fit within. Move around the photo to make sure nothing vital gets cut off, or to leave room for the text to be readable. Click off of the photo to apply the changes.
When you’re done, go to Slide Master>Exit Master view to return to your presentation. If your changes haven’t been updated, you may need to use the drop down for Layout and choose the slide with your new image.
Thank you and good luck.