Maps image to existing palette producing image in ifIndex8 format.

function MapImageToPalette(var Image: TImageData; 
                           Pal: PPalette32; 
                           Entries: LongInt): Boolean;

Imaging (at line 187)

Pal must be allocated to at least Entries * SizeOf(TColor32Rec) bytes. As resulting image is in 8bit indexed format Entries must be lower or equal to 256.