Hi there and welcome!![]()
What you describe is a common issue with repainting in any flight sim.
What causes it is the limited resolution of the repaint itself. Even though the repaint in Photoshop may be 1024 or higher, the mapping on the model might make a small area of that canvas stretch to cover a large area of the plane (and a commonly bad place is along the top and bottom of the fuselage, where stretching may occur). And the closer you zoom in in FS, the more the resolution of the canvas will show up, as in your picture, as jaggy edges.
Hopefully I've explained that clearly. It's no fault of your own, FS, or Photoshop, but rather a general limit of current game texture technology.