The Image Sizing field lets you control the relationship between the current image's width and height on the page layout and the dimensions of the original image. Click the drop-down arrow and select from the following sizing options:

Auto Height When this option is selected, adjusting the width of the image causes Tabs3 to automatically adjust the image's height in such a way as to maintain the aspect of the original image.
Auto Width When this option is selected, adjusting the height of the image causes Tabs3 to automatically adjust the image's width in such a way as to maintain the aspect of the original image.
Stretch Select this option if you want the image's width and height stretched or compressed to fit the dimensions specified in the Width and Height fields.