Is it possible to shrink backgroung image by converting it to indexed color?

Hello. I use image as background (with numbers and ticks etc.), but according to memory analysis it takes a lots of space, 78kB. Even if I convert PNG image to indexed 4bit color and it has just 5kB as file, it seems that when it is drawn in the watch it gets converted to RGB and still has 78kB in memory.

Is there any way to draw with less bit depth, so background image does not to be converted to RGB? I am using just about 6 colors in my watch face.

Thank you, Jan