In this particular case it's probably because it's just a screenshot of the 4th Ending theme which had then been upscaled immensely to this pointlessly bloated 45 MB png file.
Now if this was a scan from an art book, taken from an official source like their website, or if it was shared by the actual artist on their Twitter/Pixiv, this post would fly through the queue and get accepted in a heartbeat.
But in this current state without a real source, unlikely.
