The wallpaper consists of multiple strips of wallpaper. These are cut into lanes. The wallpaper base is always white. This means that with a dark image, wallpaper seams will always be more visible than with images that are light. This also has to do with how well the wallpaper is applied.
If you have chosen a dark wallpaper, keep the surface in mind. Perhaps it is wise to first provide the surface with a color that matches the wallpaper, so that in case of shrinkage, a white background is not immediately visible.