If Poser won't render over 300ppi just make your image larger and then change the resolution in your image editor without resampling. However 300ppi is probably high enough for most purposes (printed pictures are usually that or even under except for very fancy stuff, fashion plates say). Printing at a higher dpi than the image's ppi lets the printer use more than one dot to get the colour right, since most devices can't produce anything approaching 16m colours with one dot.