According to a recent definition (within the last few months) of a picture storybook, it is for 5-8 year olds, is heavily illustrated with a more complicated plot than a PB, and tells a story through lots of action.
There was no mention of word count, as I recall, but if you have a plot that is more complicated than a PB, it makes sense, in my mind, that the word count would have to be higher than the PB sweet spot--meaning up to 1,000--but that's just my thought.
Don't have any solid examples in my head...sorry. Examples of them would be older, yes?, since pubs have been shying away from them?