I hope it is of some use to you Issabella.
Sadly I can find no easy answer to fully automating the process using Gimp scripting tools.
Of course you do build up a feel for how certain custom fonts and decorative finishes can be split up.
Some textures and effects compress much better than others.
I did a little test based on your Longdom custom font using the same edge with a two tone inner panel given a quick shine effect:

The file info below shows that the uncompressed file is not much smaller (4 Mb) than the original but it has a much better compression rate - almost halving the size of the file.
