“…they are saved as 72 dpi jpegs.”

Jpeg is a “Lossy” format intended for web and screen viewing, but is a poor choice for printing.

Save your work in a “Loss-less” format like Tiff for printing (personally I use PDF).

 

Also 1200x1200 @72dpi is only giving you a 4x4 inch print at printer resolution (300dpi).

If you are printing larger than that, it isn’t going to look very good.

If you render your image at 2400x2400 @300dpi in Poser, you will get an 8x8 inch print that fits nicely on your size printing paper.

Chuck