Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
I've not posted any maps, but when I make maps I just use general purpose graphics software. GIMP and Inkscape are free equivalents of Adobe's Photoshop and Illustrator respectively that are more than powerful enough to make maps. There's also full-on digital painting software like Krita (also free).