Prevent Stretching of Image on Material Assigned to a 3D Plane?

I have a Unity 3D app with a bunch of materials and different jpegs assigned to them. I also have a plane with a material assigned to it. Through a C# script, I switch out the material assigned to it. The effect is that my app shows an image, one at a time and the image changes after a period of time and shows another one.

The issue I have is that depending on the shape of the jpeg file, my image displayed on my plane gets skewed. So if my plane is square, but my image file is rectangle, then the tops and bottoms of my image get stretched to fill out the plane. I don't want this. I want the image displayed just once without any stretching. How do I do this?

This is what I am currently doing:

1. Create a 3D Plane by: Game Object -> 3D -> Plane.

2. Add it to the scene, and scale it appropriately

3. Create -> Material. Done 10 times

4. Drag 10 jpgs into editor

5. Click Material. Click Albedo. Choose jpeg file. Done 10 times, one for each material/jpg

6. Drag one of the Materials onto plane

7. Add the following code to script, which happens at different events in time: Renderer myRenderer = myPlane.GetComponent(); myRenderer.material = aDifferentMaterial;

What settings should I change to prevent the stretching?

THANKS!!!