(1) In PowerPoint, click Print (Command + P), and then choose "Save as PostScript..."
(2) Open the PostScript (.ps) file in Preview, and Export it as a TIFF.
(3) Choose your favourite cropping software to crop the TIFF, which should be 300 dpi, as you like.
**Be sure that in PowerPoint you use the Insert tool to place images. Do not drag/drop or copy/paste images into the file, because this results in a 72 DPI image.