PowerPoint files with images can get very large. As soon as a file exceeds 10MB, it becomes difficult to collaborate on it via email. This probably one of the main reasons office collaboration will ultimately go into the cloud, but before that time arrives we need to deal with the current situation.
You can find the standard compression options in the format menu after you have clicked an image. Sometimes, more brutal force is required though. Somehow, if you right click an image in PowerPoint, save it is a JPG, delete it, and then copy paste it back in, the files size has shrunk a lot.
In the heat of CTRL-C, CTRL-V work, PowerPoint sometimes puts in images as bitmaps or PNG files that take up a lot of space. This trick trims them down again.
Be aware that compressing files hurts the quality of the images. So if this is a presentation destined for a huge screen at an important conference, keep the original photos somewhere in a safe place in order to be able to re-construct the full size version once you have agreed on the final document with your team.