you 2 look adorable EDIT: welp seeing you already choose one I'm not gonna waiste my time to make it transparent
Paint.NET allows you to select a region and cut it, giving transparency with the magic wand tool. Be sure to set prejudice to a low number so that you dont select anything else except for the background to be made transparent.
You will have to use a tool such as Paint.NET or photoshop, you can't make transparent images in paint.