I know of a way, but it requires an additional step. If you have a PDF printer installed which can save to PNG, just use that. If not, you can get
PDFCreator for free.
Select with marquee tool, go to file > print. Choose PDFCreator as your printer and print. Once the PDFCreator dialog box comes up, choose "PNG" under "profile" and click save. Enter filename and location and you're done.
note: to make sure you get good resolution PNGs, use large paper size, for some reason the dpi option isn't working on my machine. you can get really high resolution PNGs, something the print-screen just can't match. You can use this method to get a PNG from
any program which has the ability to print.