How to not compress inserted pictures in Powerpoint 2011 for Mac?
In Word and PowerPoint for Mac 2011, you can change the compression behaviour for a placed bitmap image via the Format Picture ribbon. Double-click the image to bring that ribbon to the front, then click the Compress button and choose the option Keep Current Resolution. Or, choose File > Reduce Size to bring up the dialog box. This needs to be done before the presentation is saved with the newly placed image.

It's different for vector graphics from PDF files, which PowerPoint will simply convert into bitmap graphics and compress as soon as the presentation is saved with the newly placed PDF file. This can be avoided in MS Office by using vector graphics in the .EPS file format or in Microsoft's own .EMF format, which can be exported from most vector graphics editors, e.g., Inkscape, OpenOffice Draw, Adobe Illustrator, etc. (Beware: PDFs exported from bitmap editors like GIMP, Photoshop, or Pixelmator are not vector graphics images.)