When you apply an image onto an object, by default the program applies the image proportionally onto the object. This means, if you apply a 200x100 image onto a 200x200 plane, the image will not map onto the plane perfectly. The image will still be proportional to how it looks originally and no horizontal or vertical stretching will be applied. However, with "UV (Proportional)" mapping, if the size of the image does not fit the size of the object, then part of the image will be cropped off.
If you want to change the mapping technique, first select the object. Then, go into the Material Docks panel. Select the material that is applied, and you will see additional controls towards the bottom. In the Mapping menu, you can switch from UV (proportional) to UV (fit), etc for different mapping techniques.
Below is a link that will go over how to get to the mapping controls.