Attached Link: http://www.soft-rabbit.com/

What version of Poser is it? On Windows, Poser 4 uses RSR files, whereas Pro Pack and Poser 5 use PNGs. There are utilities available for making RSRs from a regular image file that you can edit to suit yourself; but the Mac situation is different I believe. I've added to your post subject to attract the right sort of person in here to give a better answer. :-) I suspect MartinC's Maconverter may do it for you, but not being a Mac user I don't know.