The options argument may be a mapping containing zero or more encoding options:

normal options:
"alpha":image object
Use this image as mask
(Note: ILBM mask is boolean.
The values are calculated by (r+2g+b)/4>=128.)

"palette":colortable object
Use this as palette for pseudocolor encoding

Result is a mapping,

([
"image": object image,

... more ...
])

image is the stored image.

({int xsize,int ysize,      // 0: size of image drawing area
string bitmapheader,      // 2: BMHD chunk
void|string colortable,   // 3: opt. colortable chunk (CMAP)
void|string colortable,   // 4: opt. colormode chunk (CAMG)
string body,              // 5: BODY chunk
mapping more_chunks})     // 6: mapping with other chunks