(Picture by Dakan. A member of Crimson Creations.)
This picture is particularly impressive. But not for its appearance in absolute terms. There are prettier paintings and photographs out there.
But this wasn't made with a graphics program, or with paint. Rather, it was done with the Warcraft 3 level editor.
The way the editor works is you can assign various tiles (terrain types like grass, ice, snow, dirt) to vertices on the map (which is a square grid). You can place units (monsters, heroes, items, buildings), and doodads (scenary, gates, bridges). And you can do programming stuff.
There's also cliffs (including adding water) and raise/lower. Raise/lower changes the height of vertices on the map. Cliffs are generally kinda ugly and you can't have a sheer face more than 2 cliff levels high. If you try for more you'll get a pyramid (you can have cliffs on cliffs, just not more than 2 levels right on the edge). Pyramids don't make impressive mountains.
So the point is the warcraft editor is very limited in what it can do. To make something pretty you can basically put down a landscape, change the height of the vertices (which, btw, you have to hack if you want slopes over 50 degrees), and place some doodads from the maybe 700 or so premade models. That's it.
(Keep in mind those models include the ones for snowy areas, for grassy areas, for tropical areas, for castles, for cities, for dungeons, for caverns, and for deserts ... so for any given type of location only a small fraction are appropriate.)
The result is that most maps are kinda ugly. No offense to the creators, but they look like large patches of dirt with a few rocks placed on them, then a patch of grass with some trees on it, etc... Which is just what you'd expect given the way map creation works.
However, there are a few maps that don't look that way. So that's the first reason the picture is impressive: it makes superb use of a highly limited set of tools.
Changing focus, imagine playing a computer game that looked as pretty as the picture everywhere you went. And imagine it wasn't just like that with one pre-made story. But rather, anyone could create more scenes with different art of equal quality, without having to be any good at art.
It's (relatively) easy to draw one pretty picture. And it's (relatively) easy to draw some simple, reusable pictures. But pretty pictures tend not to be very reusable. Like this picture for example. It could be the background artwork in one scene in a game, but if you just had the image, there'd be no way to reuse it well. Its beauty is it's entirely made up of reusable pieces.
It's not just a picture. As a picture it's OK. It's a warcraft map with specifications for where different warcraft components should go, and from the specifications emerges a picture.
And the last impressive part is that the picture represents a playable area of a map. Warcraft heroes could walk around in it as is.