Many of my students have now completed their eBooks and have them for sale online. When the eBook is completely finished, a cover and some graphics will make all the difference in how well in does in the online marketplace.
I have recommended Vaughn Davidson, from Killer Covers, for quite a while now. He and his team of designers understand what you will need for your eBook, and will create a design to your specifications. Their prices are reasonable and the turnaround time is just a few days or so, depending on what you are having them do.
The alternative is to leave your eBook without a cover or any graphics, but that is not appealing to those who care about the visual effect. You can also use some clip art, as long as you own the rights to it. Both of these choices will make your writing appear homemade. If you want a professional look and feel, go with Vaughn or someone else who has done work for internet marketers. Your profits will be higher if you make a more professional presentation.