Nice video - added to Video tutorials on module development
Everyone, please feel free to add such videos to that page - it will help everyone find the information easier and faster.
Doing graphics is generally tedious - which is why I try to avoid it 
(I tend to rely on LaTeX - with my package wargame to get the graphics right).
With Gimp it is possible to set-up automatised workflows if one is doing the same thing over and over again. It is a little technical to do, but will pay off in the long run. For example, inspired by a recent post on BGG, I made a small plug-in that will take a multi-layer Gimp image and make a draft Vassal module from it:
Another tool I like to use is ImageMagick which allows one to script common image operations. For example, the script - shameless self-plug -
encodes a lot of common piece image operations, such as rounding corners, adding drop shadows, adding a border, adding a bevel effect, and so on.
Another nice tool is Inkscape - for drawing Scalable Vector Graphics (SVG). It can likewise also be scripted - for example, the Counter Sheet Extension by Pelle Nilsson can help make counters from simple input. Pelle has also make a Hex map extension for Inkscape, but I believe it is less mature and less maintained.
Yours,
Christian