Look at CopyRect for details.

function ImCopyRect(const SrcImage: TImageData; 
                    SrcX: LongInt; 
                    SrcY: LongInt; 
                    Width: LongInt; 
                    Height: LongInt; 
                    var DstImage: TImageData; 
                    DstX: LongInt; 
                    DstY: LongInt): Boolean; cdecl;

ImagingExport (at line 137)