First of all, welcome. Second, listen to RobA & RPMiller--those guys definitely know what they're talking about

IMHO, Paint is not going to yield much better results than what you have here; I think RobA's right: think about your ultimate goal. If you just want to tweak a little, this would be fine.

ON the other hand, RP's right that the GIMP is a great way to go--IF you plan to play with more maps than just this & IF it sounds like fun to learn a serious graphics program. I myself have & mainly use Adobe Photoshop, and it's a never ending learning experience, which attracts me, despite the major learning curve!

Now, if you don't plan to do much with maps/graphics, then you could also consider software specifically for mapping, which should allow you to improve this map with ease--albeit with less flexibility than a more general graphics program (Campaign Cartographer 3 may be the best for this type of map; it's pretty easy, fairly flexible, and with strong community support; there's even a free demo for it here.)

Once again, welcome to the Guild, & keep us updated on your progress!