Ellen Finkelstein has some suggestions to make your PowerPoint files more manageable.
Save the file under a new name
PowerPoint remembers all your actions in a session so that you can undo them. Saving under another name discards this information. For some reason, this works better than closing and opening the same file.
Convert the image file type
The image file type makes a huge difference. In my tests of a photo, JPEG files were the smallest by far. (GIF files are also small, but are not suitable for most photographs because they don't support enough colors.) I took an image and Microsoft Office Picture Manager to convert it.