[not strictly a VASSAL inquiry]
Many games have counters that have that cool “3D” look with drop shadows etc. Is there a simple way to create that in GIMP? (or better yet within VASSAL?)
My counter in question is a circle, and looking at the VASSAL design layout it seems like for square only…
I heartily recommend ImageMagick which can do dropshadows and dozens of other things, including nicely clipped counters as shown below:
Shadow on the right-hand side counter was generated with ImageMaick commands:
^( +clone -background #11111e -shadow 60x0+4+4 ^) +swap -background none -layers merge+1 on Imagemagick. I run all my artwork through the following, which is generated from an excel spreadsheet
convert .\Orig\us-ltc-maloney.png -shave 2x2 +repage -resize 75x75 -raise 2x2 +dither -colors 256 -strip .\Small\us-ltc-maloney.png
-shave 2x2 = Shave 2 pixels off the outer edge to clean off border pixels from borders in the artwork
-repage = reduce actual image size to new effective size
-raise 2x2 = Create a nice 2 pixel beveled border
+dither = Turn on dithering
-colors 256 = reduce to a 256 color indexed image
-strip = remove the png extra info and cruft
The reduces production ready artwork at 287.5 pixels per counter to the 75x75 size I use in my module.
I wish tools like this existed 12 years ago before I was forced to become an Adobe acolyte - a company I despise but sadly am too invested in their product to walk away…
Imagemagick initial release was 1990. It was the first tool I started using after getting to Vassal last century 
Followed soon after by The Gimp. Never found a need for anything else.
Thanks, will investigate
I use InkScape for this, but ImageMagick is great as well (just personally not versed in it).