I just went to Smashwords to check on NLM which has been pending for 2 weeks (now they have a problem with the way the copyright was phrased) and I unfortunately looked at the recently published books.
First rule--if you're not a pretty good artist, don't attempt drawing the cover illustration yourself.
I don't know what more to say about it. There's nothing that screams amateur more. If you can't find or afford an artist to work with, find a photo or vector art.
There are a number of stock photo sites where you can purchase the rights for a very minimal amount of money--and I mean under $20. Isn't it worth $20 or $12 to have decent art for the cover?
Free, in public domain images-- Public-domain-photos
5 images for $50--Shutterstock
Even less expensive than shutterstock-- Fotolia
Go to Flickr and look around. If you see something you like, approach the photographer and make an offer.
Deviantart is a terrific site not only to get ideas (plus lots of free brushes, gradients, backgrounds and hints for photoshop) but to see what people are doing. Most of it is pretty darn good, some of it not so much. Go there and look around. If you see something, the artist or photographer may be perfectly willing to let you use it for a credit or a small fee.
You are not doing yourself and the community of authopreneurs any favors by being unprofessional.
I understand how great it is to think you're going to be published but once you finish the writing of the book, you have to switch hats. You must be dispassionate, even cruel, to your work, because if you don't cross the threshold into professionalism, other people can and will be very cruel in the assessment of your efforts. You invested a huge amount of time, effort and emotion in your book. Don't put it up for derision because you didn't take time to spell check or get a sharp, eye-catching cover.