Probably not a php question, but here goes..how do I convert pixels to page units?

I have a pdf class that has a function that allows you to add an image to the pdf your creating. The problem is that the function requires that the height and width of the image be specified in page units. Well I know what the pixel height and width are of my image, how do I convert it over to page units so that the image displays correctly on the pdf? I just need the equation if you guys can think of one. THanks Brent