Thanks Arilayna & Nian
Brushes are proving tricky. Just found that if I increase the contrast before converting the pic then they stand out much better. And increasing the resolution allows me to keep detail the more I zoom in. Having said that the image is already 342mb and my pooter is having a siezure every time I try to do just an incy bit of rendering!

Think I'll probably end up dividing the main map to work on sections then combine it when I'm finished