With new version of Microsoft Office, extracting images from PowerPoint presentations is easy — all you need to do is change the extension of your documents.
Sometimes you embed an image in a document and then accidentally delete the original (or need access to the full-size image on a different computer), but unfortunately there's no built-in feature of Office to extract it for you. However, since all of Office's XML formats (docx, pptx, etc.) are essentially just archives, you can just unzip them to access the original version of any embedded image.
The easiest way to do this is to change the document's file extension to .zip in either Windows or OS X. Then, you can open up the archive and browse to the contents. Images will be in a folder called media, stored within a folder titled with the abbreviated name of the application (for instance, images would be in word > media for Word, ppt > media for PowerPoint, and so on). You can then just copy the images to the folder you want and you'll have a full size version of the original photo. Note that even if the document is saved in the older .doc or .ppt form, you can open it in Office, re-save it as a .docx or .pptx and the above process will still work.