Asda won't print my cake! Any Copyright Law experts here?

It's quite simple. Whoever took the photo owns the copyright, full stop. That person may have given a licence to a third party to use the image. Either of these parties has a right to sue anyone who uses it without permission. It's unlikely anyone would find out that their image was being stolen but an organisation like Asda must be seen to be obeying the law.

I was a pro photographer before I retired & it wasn't unusual for people to download watermarked photos from my website. I could, in theory, have sued them but it would never have been worth it.

The likes of Getty Images have been known to sue individuals who have used their images on Facebook & websites. They are quite ruthless & would certainly have a go at Asda if they found out, even after the evidence was down the toilet.