This function simply takes a surface and stretches it over the given width and height so that it occupies the area. As with draw_surface you can specify a surface and then the x / y position in the room for the surface to be drawn at and finally a width and a height (which must be pixel values).
draw_surface_stretched(id, x, y, w, h);
|id||The unique ID value of the surface to draw.|
|x||The x position of where to draw the surface.|
|y||The y position of where to draw the surface.|
|w||The width at which to draw the surface.|
|h||The height at which to draw the surface.|
draw_surface_stretched(surf, 10, 10, 100, 100);
This will draw the surface indexed in the variable "surf" with its left corner at (10,10). Its width and height are both set to 100, which is how much space it will occupy regardless of the surface's actual width and height.