I seem to remember that it was highly recommended not to scale images in PM...
It was the shift key. Check out your keyboard, or if you are running on something like virtual box/ windows xp mode, it can mess with your keyboard storkes.
You can always fix the ratio with the contol pallete
Claudio, you will always need to scale images. While I go back and resize images to 100% of the final placed size, the ability to scale images is essential in a page layout program.