I don't think it unrankable, but it may affect playing, or annoy players. In fact, I haven't played any beatmap whose BG has a watermark. But when I find a BG for a map, I always find those picture has been watermarked.
Should I worry about a BG's watermark? What's your attitude to a watermarked BG?
Should I worry about a BG's watermark? What's your attitude to a watermarked BG?