Well that depends what you want to do. If you want to setup an online equivalent of photoshop then you've got a lot to learn.
If you just want to create captcha images then you can find a class at white hat web design that'll do the job nicely.
It really depends what you need to know. I've never bothered learning to deal with images in php as I don't need it or ever intend to deal with images. Should the time ever come that I need to overly watermarks etc, I'm sure there will be a class I can use, hack, customise etc.