(1) Start Fireworks

(2) New Document.  Canvas size doesn't matter, resolution 96 pixels per inch.  Canvas color transparent.

(3) Find an image to work with.  Open  Click IMAGES, right about the search textbox.  Type the name of what you want to search for.  

(4) When you find a picture you'd like to work with, right click it, choose copy.  Switch back to Fireworks.  Edit>Paste.  

(5)  Switch to Preview mode.

(6) In the Optimize panel (far right side of Fireworks), switch to GIF and reduce the colors to 4.

(7) You can't copy & paste this one the same as everything else.  File>Export;  use Posterize as your filename.

(8) How to get this posterized picture into PowerPoint: In PowerPoint select Insert>Picture>From File

Here's a well known example of Posterize.