Only certain image types can be optimized and they have to be inserted onto the slide in a way that makes them editable by PowerPoint. Don't copy and paste or drag images in.
Inserting the image
Use the Insert menu, Picture command, and insert either from the Clip Art task pane or from a file, scanner, or camera. This stores the image natively in PowerPoint; whereas, if you do a copy and paste, PowerPoint treats the image as an object, which it cannot optimize.
Ineligible file types
Pictures you can't optimize have a drawing type of format ("vector") and have file extensions such as WMF, EMF, CDR, and EPS.
Eligible file types
High resolution images such as photographs are good candidates for optimization. Eligible file types include PNG, JPEG, TIFF, BMP, and GIF.
Tip If you use Clip Art to insert the picture, restrict your search to include only the "Photographs" media type because, for the most part, other types of clips can't be optimized.