Someone mentioned Inkscape. It is FREE and it will allow you to import images. All you have to do is to go to Google, click on the IMAGES link and type in something like overland mapping. You will get tons of images that you can use. Also try the +FREE clipart rivers (or mountains, valleys, forests, etc....). You will get tons of images. Anyway, Inkscape allows you to use everything as an object. There is also DIA (or is it DAI?). This is a CAD program that also will allow you to import images. Inkscape is more like an art program but with the two you can create anything. What I do on my FRP BBS is I do everything in Visio and then for my users I do the same design in Inkscape and post that (not always but most of the time). This allows the players in my game to move their figures around on the map to show me where they want to stand in relation to the monsters.

I've also used Screen Monkey from NBOS. It is ok but it lacks a lot of features like what Roll20 has. Last, but not least - there is Obsidian Portal. All of these are browser based programs mainly for playing an RPG on. But you can also use them to build maps which is why I even mention them. But for most people starting out I'd recommend Inkscape just to get started. Once you've exceeded what Inkscape can do you can then look into CC3, Fractal Mapper, etc....