Yes, it is always a good idea to get the book illustrated first especially if it is a short story. Some people get by by taking pictures though. You can get an agent to try and sell your story to a publishing company in order that they have their own publishing done, but you don't have to do that. There are a number of self-publishers out there where you can pick an illustrator from them and then have your books bought singularly from their company. In that way, you are not just making royalty, but you get to pick the price of the book. Before anything, I would start off with seeing what each publishing company is looking for (like the idea of SEO) and work your way from there.