You may link to the picture on your disk instead of physically inserting it.
When you bring up the Insert Picture dialog box, either by clicking on the Pictures button in a Content placeholder, or by directly selecting the Insert tab | Pictures button, do not blindly click the Insert button after selecting the picture to be inserted.
You will find a down-arrow next to the Insert button, highlighted in red :
Click this down-arrow to open a small menu where you may choose Link to File :
This will reduce the size of your presentation since PowerPoint only saves the path and name of your picture rather than saving the entire picture. Also, if the linked picture is edited outside PowerPoint, the change will be reflected within the presentation automatically as long as the file name and location is not changed.
The downside is that if the original picture is deleted or renamed, you will see a small red X icon indicating that the linked picture cannot be displayed.
You will need to always copy the picture used to the an identical folder as the one containing your presentation when taking the presentation to another computer, or you will need to re-link the image.