in your main script set d3d_autotransparenby to on. then make the areas you want transparent an off color from the rest of the image(if it is grass set this color to R+8, G+8,B+8...lighter or darker.) also paint the very top left pixel of the image this color, and you should be set, I recomend reading the manual about this setting, as it may explain it better than me. By using it you can use 24 bit images as opposed to 32.