XnView Software

When I try this:
>nconvert -canvas 100 100 center -bgcolor 255 255 255 856982.jpg
my image gets fully resized instead of just the canvas enlarged.
Does anybody know what I am doing wrong.

I now tried this export from Xnview:
nconvert -npcd 2 -size 256x256+0 -ctype grey -corder inter -out jpeg -o ...

I'm trying to use the SDK in Xbase++.
When I go through the documentation I find different structs for :

GFL_BITMAP, GFL_SAVE_CALLBACKS, GFL_SAVE_PARAMS

Can anybody tell me the exact current structure of these types?
At the moment I get wrong datatype errors.
And their correct datatypes as well ...