You would crop the picture before making it transparent. The way you crop depends on what program you're working in. In Paint, there are little blue (or white) squares in the corners that you can drag inwards to crop. In programs like GIMP, you can specify the dimensions of the document before creating it.
As for making it transparent, there are a few different ways you can go about that, depending on the programs you have. If you like GIMP, [
here's] a tut for that.
Or Paint.NET. Or PhotoShop. Or you could simply use Paint and the website iaza.com ([
here's] a tut for that). I like that last method the best, and I find it fairly quick and easy.
You also have the alternative of making a request in the
Digital Requests section so someone else can do it for you.