Look at SplitImage for details.

function ImSplitImage(var Image: TImageData; 
                      var Chunks: TImageDataList; 
                      ChunkWidth: LongInt; 
                      ChunkHeight: LongInt; 
                      var XChunks: LongInt; 
                      var YChunks: LongInt; 
                      PreserveSize: Boolean; 
                      Fill: Pointer): Boolean; cdecl;

ImagingExport (at line 127)