I use Paint.net. Has some more capabilities and is free. Also, I would add more stuff, make your own designs in some cases instead of just downloading designs. Another tip, Make sure stuff is aligned. Lastly, with flames, a lot of premade designs have some sort of flat end, where the flames would start, make sure that end doesn't have space below it on the truck, with yours, the Green flames would need to be moved down farther just till it hits the black background, which looks much nicer.
@Snake_Playz https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwj1oeax7M3VAhXkDsAKHZSmBuYQFgg7MAI&url=https://wiki.beamng.com/Skin_Creation_Megapage&usg=AFQjCNF8QG6mRwFkYbw8b6orXBpr4kKkcQ
