(Web) design is hard. For us programmers, it's a pain in the ass, and when it comes to create UI elements for a (web) app, we are stuck with two choices: 1. create something very basic, or 2. spend countless hours searching the web for Photoshop tutorials on how to create a bubble or depth effect, then spend even-more-countless hours fighting with Photoshop to implement it, and finally, maybe adapt the image with som…
I suggest to add next two features which can be essential for some scenarios: 1) Add way to choose background color (i.e. page color) for the formats that don't support transparency (gif and jpeg). PNG gradient transparency is not supported in IE6 and quite often there is a need to render button edges with background color preset because of that. 2) Pre-render images for other button states: hoovered and pressed.
And afterthought: consider supporting iPhone-style buttons (i.e. like big red "delete contact" button and small blue buttons), I'm sure a lot of people are looking for them.