No, you don't need a horizon image to show mountains in the distance:
You need a terrain and a texture to create a mountain – but you don't need distance.
You can use almost any terrain to create an illusion of distant mountains – it's just a matter of dimensions: height, depth and width.
3D is all about illusions, whether you use a screen or an object to create them.
We are pre-programmed by nature – we KNOW that mountains are big,
even when they are small and close.
Here is a tree, made in Poser with a group of cloth-planes.
Un-textured it looks nothing like a tree, but textured with an image of ferns, it suddenly creates an illusion.
