Unfortunately the existing GIMP brush engine is quite limited when compared to Photoshop's. The only jitter options are for location (not size, rotation, or opacity). The latest GIMP supports dynamic brush sizing and selecting colour from a palette, however, so these should all help...

Because I like the technique and want to replicate it in GIMP I am playing with some work-arounds, including the use of a script I posted about previously for creating "rotating" brush pipes for GIMP. Put simply, I plan to make a custom brush for drawing tiles, by manually creating the various jitters. Hopefully I can get a brush up in a few days.

-Rob A>