PowerPoint distorts the aspect ratio of images when you use them to fill a shape. To work around the issue, you need to crop the picture in the aspect ratio of your target shape. In the example of the circle below, that is a square.

I use PhotoShop to crop my pictures. You can also use the PowerPoint crop function itself and right-click, save as image.

Click "view post" to read the full blog entry. If you liked this post, why not subscribe to daily updates about presentation design via email? Just blog posts, no spam. You can find Jan Schultink on Twitter here.