Sorry for being off-topic. I recently discovered GraphicsGale had the custom brush feature (alas). For anyone in doubt or frustration, I quote directly from the help file:
Reload Pen/Tile
Reloads pens and tiles user made.
Pens and tiles must be BMP file, and must be placed in the folder named "pen" in the following folder.
WindowsXP: C:\Documents and Settings\(Windows user name)\Application Data\Humanbalance\GraphicsGale
WindowsVista: Desktop->(Windows user name)->AppData->Roaming->Humanbalance->GraphicsGale
Windows98: C:\WINDOWS\Application Data\Humanbalance\GraphicsGale
Besides, the BMP must be a monochrome(1bit), the size of a pen must be 32x32, and the size of a tile must be 16x16.
In a pen, white area will put a color in the canvas. In a tile, white area will be replaced with the first color, and black area will be replaced with the second color.