This video explains why you should be using GIMP for maps. With it, you can open GeoTIFFs and make changes (color, exposure, etc.) and export the file with the geo metadata intact. This is a HUGE time saver and allows you to modify popular base maps, including Natural Earth files. It also provides an easy way to blur DEMs to get a softer look for shaded relief, a technique that is difficult to achieve any other way.