There are a couple of ways to obtain this, basically the carrier.nut image size is controlled by carrier width and height, number of tiles and padding. In your case you only have 1 tile so width and height directly apply to the image, while padding creates a blank border around that image.
So solution 1, if your images all share the same height, you can create your carrier as large as the screen and as high as desired, set to keep image aspect ratio. Let's say screen is 640x480 and images are 300x200, you define your carrier as 640x200, and you should be good to go. This will make your images all the same size, or all the same height if they are not all at the same proportion.
If your images are not at the same proportion, and you want to have them all at the same width, you have to play a bit with padding, so you create a carrier that has a 640xdesired.carrier.height and add padding so that the actual image width is the one you desire. I don't remember how padding is placed but I think it's half on every frame so in your case to have a 300 pixel image on a 640 screen your padding should be 340 pixels.