Some authors go to the extent of designing a cover for their book when sending in a proposal. This can help the editor to imagine your book in print and how attractive it would look, but only if your design is of a professional standard. If you've never designed a book cover it might be advisable to avoid attempting one to send in with your idea. Instead, you could commission a design from a local graphic designer or art student. If the design turns out to be stunning, giving the book a similar style to a bestseller in its genre, then things could start to go your way more easily. You should supply the designer with any high quality photos or artwork you possess that might be suitable, advise them on the target audience and show them similar book covers that you like. This will help them to understand the style of drawing or design they should aim for.
A picture of your book cover is not essential, since most books that are published do not come from illustrated proposals, and the wrong kind of design can actually put editors off. But if you can get a design that excites a publisher in the same way that customers in bookshops get excited by attractive designs that make them want to buy books, then you're a little further on your way towards being a published author.
Even publishers judge books by their covers.