The worlds are square, and so are their textures and it's not possible to have ice 'caps' per say.
To make it look like a sphere, the square is first pinched into a circle shape (more strongly towards the edges to avoid obvious 'edges' on the diagonals where the distortion is strongest) and then warped to make it look like a sphere rather than a circle, and finally uv-scrolled to 'rotate' the planet.
(visualisation of a 20x20 square tiled planet texture going through the uv-operation)
You can make your world have ice biomes running across the middle of the world lets say ( a strip on the square) and depending on how the uv-rotation is set at that particular time it may look like a strip across the middle of the planet, or it may look like a pair of ice caps at the top and bottom.
The uv rotation is in both axes (we don't bother with an additional third axis spin) so that no part of the world is more 'prominent' than others, at some point every part of the world will be at the 'centre'