In the new WMS, each image element can contain width and height details. This information not only serves the user to show what the expected size is, but WMS is smart enough to do all the image re-sizing work for you!
When selecting the image, WMS checks the size of the image you selected. It can then tell you the image size is incorrect and offers you a chance to re-size it automatically.
Resizing creates a brand new image and based on the element settings can keep the ratio of your image.