In Poser you can output a masked (aka 'cutout') PNG which has 'invisible' pixels for the background. it's then just a question of dropping these over a backdrop in a layered Photoshop file. As of Poser 12, "SuperFly now does background transparency" and also "shadow catching".

If you are making quick Preview renders (e.g. for a comic) then have the Shadow Map size on lights up to at least 1024 (default is 512), if not 2048.  This will improve shadow quality. Regrettably it seems there's no way for a Python script to tweak these dials up with a single click. There's no hook that a script can hang onto to change "Preview Shadow Maps Size" in Poser. They have to be increased by hand. I assume that's still the case in Poser 13.

There is however a Poser script, “Change Shadow Intensity of Selected Lights”, if they are too black. There are also "Scene shadow blurring scripts for Poser", which in one click has the lights use softer ray-traced shadows, rather than the default depth-map shadows. Check out the Poser-made comics of Brian Haberlin (Sonata etc) to see how he handles shadows from Poser figures.

There's also a "render each figure/prop separately" script than can be made to work in Poser 11 and probably 12 and 13. Theoretically you could then select / fill the rendered figure shape with black, skew and distort and blur it in Photoshop, and call it 'the shadow'. I think there are also several 'long shadow' actions in Photoshop that do something similar, having a PNG cutout cast a long shadow of the cutout's shape. The advantage here is that these can be somewhat adjustable in its placement behind the character. I think that Richard Rosenman has a free one, but I could be wrong. It might be paid.

Greenscreen would though be needed to make a 2D 'fish in a bottle' Photoshop layer set made of three renders. The front and back of the bottle, and the fish (or whatever you want... octopus, pickled gnome) in the middle. In which case you slide a bright green square into the centre of the bottle, and use Primatte in Photoshop to remove the greenscreen. You'd do this if you wanted to make a kit showing commonly repeating jars and bottles, and you didn't want to have to render each and every one.

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Learn the Secrets of Poser 11 and Line-art Filters.