$WW,1$GRA graphics files are 8-bits-per-pixels but only 4-bits of color, with transparency and no palette.  Compression is the standard TempleOS$FG$ LZW compression.
$HL,1$
#define DCF_COMPRESSED		1 //This is the only saved flag.

#define TRANSPARENT		0xFF
#define BLACK			0
#define BLUE			1
#define GREEN			2
#define CYAN			3
#define RED			4
#define PURPLE			5
#define BROWN			6
#define LTGRAY			7
#define DKGRAY			8
#define LTBLUE			9
#define LTGREEN			10
#define LTCYAN			11
#define LTRED			12
#define LTPURPLE		13
#define YELLOW			14
#define WHITE			15

class GRAFile
{
  I32	width;
  I32   width_internal; //Rounded-up to multiple of 8.
  I32   height;
  I32	flags;		//DCF_COMPRESSED? See $LK,"::/Kernel/Compress.CPP"$.
  U8	body[];
};$HL,0$

See $LK,"DCSave",A="MN:DCSave"$(), $LK,"GRAWrite",A="MN:GRAWrite"$(), $LK,"DCLoad",A="MN:DCLoad"$(), and $LK,"GRARead",A="MN:GRARead"$().