Have you recently switched from 2003 or 2007 to 2010? My first thought is that you either drag and dropped or copy and pasted images into the powerpoint as links instead of insert>picture> from file. It is always best practice to do it this way and make sure that Link is not checked when you insert. See if you can check the source of some of the images and verify if they are linked to a file or embedded into the document. Linking means the project is smaller because the linked objects are saved in the root folder with the powerpoint instead of actually embedded into the presentation.