Operative system, If windows, intall pain.net, it's pretty the same, but paint.net has a bit more stuff.
Terraria Community Forums
Frous
Enjl
Guess why I'm using Pinta instead of GIMP or Photoshop. I'm so accustomed to paint.net that said other two are just absolute trash. HOWEVER. Linux gets no paint.net. So Pinta will have to do.