I still work with the 2008 Powerpoint version and was very surprised to find missing the very useful option Powerpoint Windows always had whereby a right click on any image would allow you to compress that images or all images on the presentation, so you could always add heavy images and postpone compression until the end as well as control which image to compress. But the option is gone.
Read the off targe advice on this thread and ...
1) I tried the Save As / Options / Compress Graphics File path but did me no good (as advertised later)
2) I tried looking for Keynote as suggested, but I do not have it and when I looked for it in the Apple store a $19.99 download was available but ... I needed to update my OS to OSX 10.7.4.
3) I disagree with the "go to the Microsoft forum suggestion" as the question is relevant to Mac users that use third party software (even if it is from the evil Microsoft empire)
4) I ignored other suggestions to find and install new software to accomplish this task on principle and for practical reasons as well as I need to finish a 25 slide presentation in the next few hours and I have no time to spend 3 hours buying and installing a new OS and cannot accept the proposal to install new software to solve this problem.
Bottom line is, if using Powerpoint 2008 in the Mac avoid inserting large image files to begin with.
But if you are already in the situation and do not want to start the presentation from scratch you can follow this workaround I found after a bit of experimentation.
I am not in the extreme situation described (500MB file with 100+ images), but in the much more common scenario of having 25 slides with 10 or 12 images that may each add 1 or 2 MB to the file size that bump it beyond the 2 to 3 to 5 MB range to email through corporate email servers, this will solve the problem one image at a time:
1) go to the the slides with the heaviest images and work your way down through the images until you achieve the file size you want.
2) For each image, copy it and open Preview (the program that has the picture with a child on the beach and a looking glass).
3) In Preview under File, choose "New from Clipboard". Under File, "Save As" and give it a name so you can save changes. Under Tools, "Adjust Size" and work with the (default) percent compression values.
A 50% reduction for a large image (2MB) that you are using to fill the Powerpoint slide will probably go unnoticed but will chop off a large chunk of the file size. If you are using a 2MB image to have a small rectangle in the page, use a more radical 20% compression. Play around a bit to get a feel for the amount of pixelation you can live with in your presentation vs the total file size. You can Undo (Command Z) the compression and compress with a different value and save each compressed image under different names ("image 50", "image 80", etc) to see the image file size.
4) Go back to your Powerpoint file, delete the large image and insert the compressed image. Repeat until satisfied with the total file size.
If you are doing a new presentation and have several heavy images to insert the Preview program allows you to open several images at the same time that will appear on the right hand side, you then need to select them all inside the program (command A), select Adjust Size as indicated above, select a percent compression and Save All. Images will be overwritten, so if you want to preserve the original files do this with a set of copies in a different folder.
