Working with png files in Java or C#?

Hey everyone, I've been working on a mod for minecraft and had to update to the latest version and now textures are handled differently. This isn't the reason i'm here though, I won't give all the details but basically textures went from being one large sheet image to each texture being in its own file. My mod had hundreds of seperate textures and I would have to crop each one in gimp one at a time if I did this manually.

I think it's obvious why I'm here now, my current little project is to make a program that takes a single input png file and outputs several smaller files of the input split up. I have an idea of how I want to do this, but I have no idea how to:

• open png files, i've worked with .txt files before so I would think it's a similar process
• copying from and editing images, I'm assuming png files are basically huge 2d arrays of pixels though
• create new image files for the output

I'm not asking you guys to write the program for me, I just want to know the syntax for working with png files in general. Thanks a lot guys!