This isn't what you're asking, but I wanted to make sure you were aware of what GIMP can do.
I'm a GIMP user in Linux. Instead of learning yet another application, may I suggest exporting the image differently?
In GIMP, File --> Export As --> Enter the filename. You'll get a pop-up window titled "Export image as...".
The first entry is Quality. Lowering the quality will reduce the bit size.
The trade is that reducing the number of bits reduces the image quality. You can't work around this regardless of the application used. So, stick with GIMP and experiment with the setting. After it's exported, open the file up in GIMP and compare.
From past experience, making high quality low bit images is very, very difficult to get right. It's a matter of what you'll accept for the intended purpose.