Yes, on a technical level, you probably are. The thing to remember with copyright law (or more specifically, fair use) is as it exists, there are only general guidelines for declaring what is fair use and what is a copyright violation. A judge hears from both sides on a case and then decides based on a few factors including:
-Whether you are attempting to impersonate the original author, or if it's clear that the work is derivative
-The purpose of the usage (is your intent satire or parody, or were you just lazy and didn't want to make your own works?)
-Whether you needed the original work to acheive the intent of your work
-If the purpose was for educational goals
-Whether you are charging for the work or not
After all that is said and done, the judge declares fair use or not. Given the circumstances of why and how you made that DVD cover, Lucasfilm could easily tell you to stop by sending a Cease & Desist, but they would probably not have a case for seeking restitution or other punishment.
One positive note i can pass on though is that Lucasfilm is extremely positive about fan works and the odds of you ever hearing from them over a DVD cover is extremely low.