This is an easy way to reduce image file size. What you have to do is to convert the image mode from RGB to Indexed colours.
In GIMP, click on Image > Mode > Indexed.
Finally, change the “Maximum number of colors” property value from 256 to 16 or 32 as shown in Figure 1.
Figure 1: Indexed Color Conversion
gotta to try it
gimp also available for redmond os