Hey I've been using signpicture for a while and it is amazing.
Unfortunately some servers don't let URLs be put onto signs which kinda sucks...
Would it be possible to have a folder within the signpic folder in .minecraft for images that the mod can use? This could also be useful for offline mode if you want to use an image and can't upload it.

Local files can be load from the resource pack.
For example, /assets/hoge/fuga.png is !hoge:fuga.png.
You can add images to uncompressed resource packs and use them.

I tried this for a while and it does not appear to work for me.
I created a new resource pack to put images in and tried things like:
!assets:minecraft:test.png
!pack:assets:minecraft:test.png
!resourcepacks:pack:assets:minecraft:test.png
Is there something I am missing?

You can write like this.