Hire a freelance graphic designer as a consultant to work out some ideas on what would look best for the type of CD that you're releasing. Working one-on-one with a designer will help you grasp the exact vision that's in your mind's eye. The best way to create that image is to listen to the CD. The music will paint a picture that reflects the overall feel and content.
Purchase CD label software. This is a practical and economical route, as the software usually comes with templates that can be used or modified to taste. Some of the more popular packages are Avery, Fellowes, Neato and Acoustica. The packages normally include labels for the actual CD and CD cover inserts for jewel cases, and they may allow you to import digital pictures. If you have iTunes, you can also use it to print your CD cover.
Learn how to use image editing software. By utilizing Adobe Photoshop or GIMP (which is free), you can have total control by designing everything from scratch. If you don't mind taking the time to learn, this method could produce some very high quality CD covers. Get acclimated to your software by joining graphic design forums and asking questions. Members are usually more than willing to help.