Is it better to sell a PB as a stand-alone and then mention to the editor that you would love to expand your book into a series after all contracts are signed? If you mention 'series' before contracts are signed, would that make an editor question if they should buy a book that has stand-alone potential?
If you send me a picture book I want to acquire, I'm going to acquire it. If you mention at that early date that you have ideas for second, third, and fourth books just like it, I'll explain why we're going to wait for some market feedback on the first one before we throw money into developing a sequel. I'll be nice about it, but I'll be rolling my eyes the whole time.
Lots and lots of authors are so enamored of what they've created that they envision it becoming a series. If the book-buying public turns out to be just as enamored, then great! If not, no dice.
Some types of books are best as a series, but as I mentioned before, they should have very solid market potential.