Writing Merged composite image to PSD file??

Hi,

I'm developing PSD file writer which adds layers to PSD file.
Can anyone tell me how to write merged composite image to psd file,
so that it shows composite image when i open that PSD file in other image viewer applications such as irfan viewer or Quick time picture viewer.

Currently i've been successful in exporting each image as layer. But due to some reasons merged composite doesn't show up.

Thanks in Advance,
-Suma