Inserting images in SketchUp is extremely easy. Go to File in the menu at the top and select import. From the Open dialogue box navigate to the image that you wan to import to your model. SketchUp allows you to import all of the most common image file types. Select the image you want and click open.
You will then be returned to the model space. First select where you want to drop the image. By default you will be dropping the image at its corner. If you hold control you will now have control of the center of the image. After you click once to drop it move your mouse and you will see the image change size. Depending on the direction you drag your mouse, your dimensions will reflect either the height or width. If you want a certain height for your image drag the mouse up, type the dimension you wish to use, and press enter. If you want a certain width for your image drag the mouse left or right, type the dimension you wish to use, and press enter. By default the proportions are evenly scaled. If you wish to distort the image you can do so by holding the shift button on your keyboard.