The pouch of secrets contains most of the content that I used to make the graphic pack. It's in the GIMP format, and most file contain multitudes of layers for different object, to separate foreground from background, and to create complex foreground/background graphic effects.
You can't directly use it for the game, but other people can use them to more easily add things to the graphic packs without my help.
On that subject, yes you are free to add things to the graphic pack, or make your own version of the graphic pack. Most of the art in the pack has been, borrowed, from the community, and I've created to pack for the benefit of the community. Feel free to use or improve it as you wish.
Sadly, I will no be able to update the graphic pack myself. I suffer from anxiety/dread issues, and working on this pack is one of my triggers (download GIMP and study the graphic files to quickly understand why.) I'm sorry I can no longer support the pack. I will visit/lurk once in a while, like I did for the past year, to keep up with what's happening with DF.
In the spirit of the winter season, I wish you all some happy goblin slaying and elf forest lava flooding!