That is JPEG compression. Depending on what version of Poser your using, P7 I think you set to 100% and the other versions were the other way.

It is better to save the render out in one of the lossless/ alpha channel formats such as Tiff, which I prefer or PNG or PSD? does poser export PDS? ca't remember. But this way you can import the image into your favorite paint program and do all your postwork then save out at what format and compression you want.