Not really sure what you mean by resolution of the PNG image. As the PNG format stores pixels, the resolution of an (unscaled) image only depends on the resolution of the viewing device (such as a monitor). In other words, the trivial approach to a more detailed image would be to create a larger image, with drawing coordinates, font sizes, etc. adjusted accordingly. I suppose this occurred to you already... which is why I'm asking what you mean by resolution.
If it's that the text/lines in the image look "ragged", you (or your boss) might actually want antialiasing, which makes the image appear smoother to the human eye (by reducing spatial sampling artifacts). GD offers gdAntiAliased color for drawing, but I think for text only TrueType fonts provide antialiasing (method stringFT()).
