Crop part of a TImage into another TImage


I'd like to crop a portion of a pic (the user selects which one using a
square shape) and show that portion magnified in another timage

I'm using the CopyRect method for it: I define  a trect with the position of
the shape, then I do

to copy the portion of pBmp given by MyRectDest and put it into the canvas
pBmpDest in the position given by MyRect.

But (there's always a "but"), the destination image always shows the top
left corner of the source image.

I'm doing something wrong but, what?? Please, give me your comments.

Thanks, guys. Regards from Spain