This covers basic reskinning for Paint.net and GIMP. Sorry for the abrupt ending.
Paint.net: https://www.getpaint.net/download.html
GIMP: https://www.gimp.org/
TrainzMeshViewer: http://trainz.shaneturner.co.uk/tutorials/index.php/home/utilities/pevsoft-trainz-tools/trainz-mesh-viewer-2
