Some of these download sites are a little deceptive and you need to be careful which link you click. However, I just followed Kieran's link and, while it took me around a couple more links, I did download the zip file. My antivirus said it was clean. BTW the actual download link was
http://www.dotpdn.com/downloads/pdn.html
Actually I think Paint.net is just as confusing as GIMP. All those windows all over the screen annoy me. If I had to choose one over the other I would choose GIMP.
For the original poster I suggest you also get a copy of PEV's Images2TGA which is Trainz friendly and can do some clever stuff. It is not an image editor like Paint or GIMP but can be set up to launch those programs if required. I use it a lot.
You can get Images2TGA via Shane's site at
http://trainz.shaneturner.co.uk/tutorials/index.php/home/utilities/pevsoft-trainz-tools
Also, I had to ask - is there any reason why the OP could not use PaintShed with TS06 and then import the CDP from his/her modified loco/traincar into TS10? That might be an easier option although I see she/he has already had some success.
I still have some Paintshed pdf file giving instructions. They are a bit ancient - just like me. :hehe: