Something like this:
[pascal]
var bufferbyte;
size:word;

begin
size:=imagesize(x1,y1,x2,y2);
getmem(buffer,size);
getimage(x1,y1,x2,y2,buffer^);
blockwrite(f,buffer^,size);
freemem(buffer,imagesize(x1,y1,x2,y2));
[/pascal]