Uncompressed pictures are a common cause of large PowerPoint files. When photos are added to PowerPoint slides, they're saved in their full resolution in case they need to be zoomed or cropped while editing the slide, or in case the slides will be displayed on a very large projector screen. This can make your PowerPoint file size quite large.
To reduce your file size after you're done editing photos, you'll need to compress the pictures. (This won't change their size on the screen, only their size in your file!) To do this, you'll need to have the desktop version of PowerPoint installed, since the online version doesn't yet have this capability. (Students: To install it for free on your computer, see this FAQ: Office for Free )
PowerPoint 2016 or 2013 (Windows version -- Mac is below!):
1. Click (left-click) any photo in your PowerPoint.
2. In the top menu bar, select the Format tab.
3. On the left side of the tool bar, click on Compress Pictures and a dialog box will pop up.
4. IMPORTANT: Uncheck the box for "Apply only to this picture". (You want it to apply to all!)
Make sure the box is checked for "Delete cropped areas of pictures".
5. Click OK.
6. Go to File > Save As and save a reduced copy of your file with a new name, and then compare the size of your original file with your new file. Hopefully it's a lot smaller!
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PowerPoint 2016 (Mac version): (NOT Apple Keynote!)
1. Click (left-click) any photo in your PowerPoint.
2. In the top menu bar, select File.
3. Click on Compress Pictures.
4. In the Picture Quality box, select Email (96 dpi).
5. Select "Delete cropped areas of pictures."
5. Select "All pictures in this file".
5. Click OK.
6. Go to File > Save As and save a reduced copy of your file with a new name, and then compare the size of your original file with your new file.
Microsoft offers more info here: Compressing pictures in Office for Mac
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If your new file isn't much smaller, some or all of your photos may not be compressible. The "quick and dirty" way to fix this is to do this:
First make a backup copy of your original presentation.
Then, for each slide, take a screenshot (snip) of your photo(s) in the slide (or a screenshot of the whole slide), copy the screenshot, delete the photos in that slide, and then paste the screenshot into your slide. (You may need to resize or move the inserted screenshot to the correct spot.)
Do this for each slide (one by one) and then save your file under a new name. Compare it to the original to be sure it's smaller.