Sites like MPIX and Shutterfly offer tons of templates with multiple photos, so the easiest way is to just use one of those. If you want to do it yourself, you can always make a college to your exact liking in Photoshop, then export the image and print it as a single image. When you’re making your design, make sure you use the same aspect ratio of the blanket so it doesn’t get cut off or squished during printing.