TImage differential

[syncovery]

Dear image developers,
please suggest me a simple way to compare two TBaseImage objects and get third differential image object.


my question is very relative to this post
http://galfar.vevb.net/imaging/smf/index.php/topic,274.0.html

but i want to get as output smaller image. e.g Picture 1( Desktop ) - Picture 2( Desktop + Dialog) = Picture 3( Dialog ) but small size image of the dialog only.

thanks in advance,