Is it because I've got 'Shitake' in the key words? Just the name of the mushrooms.
If this is the image you submitted, the watermark is a Bad Thing.
Thanks for your input. This was my error in the versions I posted here. The actual submitted/rejected images did not contain any watermark. I'm thinking it must have something to do with keywords.
Oh boy. I've just rechecked - the submitted images did include the watermark. Schoolboy error. Thanks for pointing it out, and apologies for wasting your time!