But doing that requires loooong rendering time so I resorted to image sequences.

When I pre-rendered by clicking the camera in the render tab, it shows the effect I wanted plus the green background but when it is finally rendered as image sequences after clicking the make movie button from the render settings, the supposedly green background turns light gray leaving its trace: a thin, broken, green outline on my model (see pic).

I wonder what am I doing wrong? How can I ensure this green background to show in my image sequences?

The 1 second video that took a long time to render but has the green background that I want:

www.youtube.com/watch

never mind. i figured it out. it appears to be poser makes my green background automatically converted into a transparent  image which explains why the image above looks like that.

It sounds like an Alpha channel thingy.

Some software will make the Alpha channel invisible when you view the image.  I know that Windows Preview does this.  

You can get rid of this by processing it through another application like Adobe After Effects or Photoshop. 

In Photoshop you would want to record an action of deleting the Alpha Channel (which can be seen under "Channels").  You can then replay this action and have it go through and process all the images for your sequence.

You will see 5 channels.  RGB, RED, GREEN, BLUE and ALPHA.

Alpha is good if you want to layer your sequence over a background image.

I'm not certain how to do this with other software, but there are ways.  Also most video editing software can be set to ignor the Alpha on import.

Hope this helps a little.