Can't help it - gotta chime in here - yes, you have it exactly right Gini. Don't get frustrated - everything is in fact theoretical until you go to output - at which point you need to have the appropriate dpi for the desired output size (which is where Tiari is going - but not taking into account that you're talking about rendering a high pixel dimension in the first place).

Yes, you render an image 2400x3000 pixels if you want to eventually print an 8x10 at 300dpi - or a 16x20 at 150dpi - see, it's the same 2400x3000 but you're only changing the real worl output for a print you will hold in your hands. Until then, it's merely pixels in a digital file and can be set to 10dpi for all that matters.

The fact that Poser renders at 72ppi is a non-issue - RHaseltine is exactly right about simply changing the "setting number" of dpi/ppi by making sure Resample is unchecked - you are never making any size changes to your original 2400x3000 image as it's still the same dimensions - in a computer until it goes out to print.
Hope that helps-
-Lew ;-)