Pictures and images take up a lot of space in Power Point presentations.  You can compress the image sizes to make your file much smaller without affecting image quality.

Using Power Point 2007 (pptx)
1. Open your Power Point presentation
2. Scroll to one of the images in your presentation and double-click an image. This will change the menu options at the top.  On the left hand side of the menu, select "Compress Pictures".
3. Click [ok] to finish process
4. Save the presentation

Look at the size of the file and proceed with uploading it if the size is less than 50MB.  Usually, a 50MB presentation with many images will shrink to 5MB.