How do I keep image aspect ratios when changing the aspect ratio of an impress slide show?

I am frequently given slide shows that are done as 4:3 to show on a 16:9 screen. I would like to change the aspect ratio of the show and of the master slide, but leave the images in their original ratio.
At present I go to format > Page and set the format to screen and then set the size to 32 x 18.
Then I look at the slides and any photo that was inserted gets stretched to a new aspect ratio.
I would rather have more white space and the images left at their original ratio.

Any clues?

When you are on Format > Page you should see a checkbox in the right area with label “Fit object to paper format”. Uncheck it.