See https://sourceforge.net/p/openil/patches/42/

This patch allows luminance BMPs to be saved as straight grayscale, saving
lots of space for colorless images.
--- src-IL/src/il_bmp.c	(revision 1662)
+++ src-IL/src/il_bmp.c	(working copy)
@@ -928,6 +928,28 @@
 		}
 	}
 
+	if((iCurImage->Format == IL_LUMINANCE) && (iCurImage->Pal.Palette == NULL))
+	{
+		// For luminance images it is necessary to generate a grayscale BGR32
+		//  color palette.  Could call iConvertImage(..., IL_COLOR_INDEX, ...)
+		//  to generate an RGB24 palette, followed by iConvertPal(..., IL_PAL_BGR32),
+		//  to convert the palette to BGR32, but it seemed faster to just
+		//  explicitely generate the correct palette.
+		
+		iCurImage->Pal.PalSize = 256*4;
+		iCurImage->Pal.PalType = IL_PAL_BGR32;
+		iCurImage->Pal.Palette = (ILubyte*)ialloc(iCurImage->Pal.PalSize);
+		
+		// Generate grayscale palette
+		for (i = 0; i < 256; i++)
+		{
+			iCurImage->Pal.Palette[i * 4] = i;
+			iCurImage->Pal.Palette[i * 4 + 1] = i;
+			iCurImage->Pal.Palette[i * 4 + 2] = i;
+			iCurImage->Pal.Palette[i * 4 + 3] = 0;
+		}
+	}
+	
 	// If the current image has a palette, take care of it
 	TempPal = &iCurImage->Pal;
 	if( iCurImage->Pal.PalSize && iCurImage->Pal.Palette && iCurImage->Pal.PalType != IL_PAL_NONE ) {
@@ -948,8 +970,8 @@
 	//BITMAPINFOHEADER, so that the written header refers to
 	//TempImage instead of the original image
 	
-	// @TODO LUMINANCE converted to BGR insteaf of beign saved to luminance
-	if (iCurImage->Format != IL_BGR && iCurImage->Format != IL_BGRA && iCurImage->Format != IL_COLOUR_INDEX) {
+	if ((iCurImage->Format != IL_BGR) && (iCurImage->Format != IL_BGRA) && 
+			(iCurImage->Format != IL_COLOUR_INDEX) && (iCurImage->Format != IL_LUMINANCE)) {
 		if (iCurImage->Format == IL_RGBA) {
 			TempImage = iConvertImage(iCurImage, IL_BGRA, IL_UNSIGNED_BYTE);
 		} else {