Any pixel editor (Paint Shop Pro, PhotoShop, Paint.NET), vector editor which supports images (XaraX, Illustrator, CorelDraw, Flash) or Text editor which supports images can be used. I typically stitch them together with Paint Shop Pro.
The reason I'm outputting a lot of small format instead of a single, big image, is that a single image requires a large amount of continuous memory, which might not be available when Rhino+Grasshopper are already using a large part of the allowed 2GB.
Of course on 64-bit systems this makes no sense, maybe I should add some logic to the exporter so that it only does the splitting when you're running Grasshopper inside a 32-bit Rhino.