I am not certain I understand what you are looking to accomplish, but here
is what I believe you are looking to do:

Option 1:
- In photoshop or other image editing program go into the
brightness/contrast, curves, or similiar tool and create a muted version of
the background graphic (enough so text will be legible on top of it). Save
out with a unique name.
- Make this image the background on the master slide.
- Insert the "full strength" version of the image onto the needed slide and
give it a FADE OUT animation.
- This should give the visual of the photo fading from a full image to a
muted image with the presentation content on it.

Option 2:
- Make the 'full strength' image the background on the Master Slide.
- Add a white rectangle autoshape that covers the entire slide.
- Go to the paint bucket and More Colors and set the white rectangle to 60%
opacity (experiment to find the best level).
- This will make the image appear muted so presentation content will be
visible.
- For the transition effect, insert the full strength image onto the needed
slide, apply a FADE OUT animation to go from full strength to muted.