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This will change every image in the PPT deck to 96 dpi from 200 dpi, effectively reducing your file size by 50%.
To further reduce your file size, open a New Folder on your Desktop. Next right click and save each photo image as a .JPEG file, into the New Folder. JPEG image sizes take up the least amount of space compared to .bmp, .png or the enormous file size .tif.
Then delete all the original photos in your slide deck. Next click insert photo, and Select All images in the folder. This will batch insert all the photos onto a slide in your deck. Then manually cut and paste each image into its original place in the deck. Finish and Save your file. Click on properties and see how much space you saved. I recently reduced an 80 MB file full of .tif images into a 2 MB file.