Tilesets are single images that contains a mosaic of regular, smaller images that have been designed to complement each other. When a tileset is loaded as such, the single tileset image is essentially ‘cut’ into a multitude of separate images that can each be used separately and repeatedly to create a much larger background image.
So for example, using this tileset:
we can easily create this sort of background image:
Tileset design can be a very rewarding, if time consuming, part of the whole game design process. Indeed, it’s almost a puzzle game in its own right, designing tiles so that they ‘plug in’ to each other, even if they are rotated…
To learn more about tileset design, this article from Gamasutra provides a good overview of the complexity involved in designing a tileset that can be used to create background terrain: Tiled Terrain.
For a slightly gentler introduction, try this tutorial on So You Want To Be A Pixel Artist?.
I’ll put together another springboard post about how to go about designing the “3D” like tiles when I’ve covered a little bit of the theory behind how it works (if you’re keen to know more now, searching for isometric pixel art should turn up something relevant!)
In the context of Game Maker, the Tileset Maker for Game Maker provides a basic environment for creating your own custom tilesets.