I am biased. I bought Paint Shop Pro in 1993 and have upgraded to each version since.
While the industry "standard" is Adobe's Photoshop, the current version of Paint Shop Pro, V.9, is, I believe, as powerful and a lot less expensive than Adobe's excellent product.
For flight simulator repaint purposes, Paint Shop Pro is more than adequate.
I use PSP7 & swear by it. Does all I will ever need & some things I will never need or understand. Ideal for repainting & there's several tutorials based on it.