I decided to set out and design a 2020 FPIU Homeland Security texture but in doing so I came across some hurdles. For one, I don't have Photoshop so I had to use GIMP as the editing software. Two, I designed the texture essentially as an addon to user @Llama's Homeland Security pack. But now I have the issue of a weird shadow and hairline seams appearing around the car. Attached is the actual .png file as well as screenshots of what I'm talking about. Any ideas of how I can fix this, thanks.
P.S any ideas of better ways to make textures would also be greatly appreciated.