Garr Reynolds is one of the greatest presentation gurus in the entire world! I subscribe to his Presentation Zen blog, and today he came out with a great post that is both simple and brilliant.
I have seen (and even done) most of these 11 mistakes that people make when using photos or clip art in a PowerPoint presentation.
This is a great lesson for all of us: http://www.presentationzen.com/presentationzen/2009/08/10-ways-to-use-images-poorly.html
My helpful tip: when resizing an image in PowerPoint, grab a corner "handle." That way you are simultaneously changing both the horizontal and vertical size of the image, and you will keep it in the proper ratio and not distort the image as Garr demonstrates.