Look at StretchRect for details.

function ImStretchRect(const SrcImage: TImageData; 
                       SrcX: LongInt; 
                       SrcY: LongInt; 
                       SrcWidth: LongInt; 
                       SrcHeight: LongInt; 
                       var DstImage: TImageData; 
                       DstX: LongInt; 
                       DstY: LongInt; 
                       DstWidth: LongInt; 
                       DstHeight: LongInt; 
                       Filter: TResizeFilter): Boolean; cdecl;

ImagingExport (at line 146)