If you're designing the cover yourself, or paying for one to be designed, you'll need to find the dimensions of the book from your site of choice. You'll need to include the front and back covers, and the spine, all as a single piece. Then, when it gives the option, you simply upload your ready-prepared cover.
If you're not using a ready-made cover you'll still need an image to go on the front of your book. Lulu allows for abstract shapes to be used instead of uploading a picture, but that's not recommended as it can turn out very generic. It's also not recommended, unless you're a professional photographer or artist, to use your own photos or art as they can look amateurish. Of course this depends on what you want from your book. If it's for personal or family use then a meaningful photo might be just what you need.
There are plenty of stock image sites that you can choose an image from, and download for a small fee. Don't be tempted to just pick one from the internet, as this is copyright infringement and exactly the same as if someone decided to use your work in their book without credit or permission. The stock image sites have licence agreements for their images, and you need to choose the licence that best suits your purpose. This isn't as difficult as it sounds as these are usually laid out fairly clearly and sites often have a FAQ explaining which rights you're buying. There's usually a limit on the number of copies of the book you can sell, but if you ever sell that many you'll be able to afford to hire someone to create a cover for your exclusive use so it's not really an issue.
There's a great deal of debate about covers in self-publishing circles. It's true that bought covers look more professional, and with a stock image you run the risk of someone else using the image, or it being a little generic for your work. I have anecdotal evidence from a friend in self-publishing that her books with bought covers sell more than those without. In this instance cash may well beget cash. Some of us simply can't afford to shell out for someone else to design a cover for a book that may never earn it back. There's no shame in that. There's also nothing to stop you going back and reissuing books with a new cover if you suddenly come into money.
Whatever you choose, all that matters is that you're happpy with your cover before you print the book.