I was trying to render a large scene that Poser didn't want to render so I tried it without shadows - it then rendered. If I now render the shadows - how do I put the two images back together - I have the GIMPand Paint.net. 
Thank you

Save the images out as PNG.  The blank areas will be, erm, blank.  Drop one layer on top of the other.  Presto!

That's assuming Gimp or Paint.net support alpha channels.

Thanks SamTherapy - but when I rendered it the background was white ;)  in the  render screen. Have to look at the .png in Irfanview  -

I tried rendering shadows only, saved it as a ping and ended up with shadows on a white background as well. When I open the image in PhotoShop it's just a one-layered png consisting of shadows against a white background. Not good.

If the drawing programs have layering effects, change the layer type to multiply or darken or overlay or some other combo. White should disappear with the listed options (but would have an effect on other options

I don't know how it wight work in the software you mention but maybe this will help and or give you some direction.

In Photoshop this is how I do it.
Bring both layers in, any format but they must be the same size/resolution to overlap properly.
As I write this I wonder, Maybe pngs won't work? Keep that in mind.

With the Shadow layer on top. It should be mostly white with some grays and darker areas, these are the actual shadows.
Now select that layer and change it to "Mulitiply".

In Photoshop, if you want a darker shadow you can duplicate that layer.
Or if you want a lighter shadow you can either reduce the opacity or use the eraser at say 23% to erase some areas.

Want to make your own shadows?
Make a new layer (on Top), also set at multiply and paint some grays.

Hope this helps.

Scott

@Paloth - that's what threw me - white background no alpha channel or whatever it's called

@gagnonrich and blizzard - thank you both multiply did the trick so I learned something new today - thank you