If your image does not have a palette of 256 colors, you can convert it to 8-bit indexed color mode with a custom palette in GIMP.
+ Open the image in GIMP.
+ Select Image > Mode > Indexed.
+ In the Indexed Color Conversion dialog, choose "Generate optimum palette" as the conversion type.
+ Under the "Maximum number of colors" option, enter "256".
+ Check the "Use custom palette" checkbox.
+ Click the "Edit palette" button.
+ In the Palette Editor dialog, click the "Import Palette" button.
+ Select "From file" and choose the palette file (palette.gpl).
+ Click "OK" to close the Palette Editor dialog.
+ Click "Convert" in the Indexed Color Conversion dialog to convert the image to indexed color mode with the custom palette.
+ Export the image in PNG format.