You want, and all you need from a Traveller perspecitve is that Black n Red Traveller logo on the front - and also dare I suggest on the back in a rectangle block and on the spine as well.
That is the iconic 'THIS IS TRAVELLER' mark. In my opnion, it is enough for anyone you want to be grabbing it on this basis seeing the side or the front or back to go 'Ooooh, Traveller'.
However, you don't want it too large. This is Prime Directive after all. Prime Directive is the setting, Traveller is the engine - so it shouldn't detract from the core fact this is Prime Directive. As a Traveller player, I want to also know as I look at it that this is not a Traveller/3rd Imperium classic setting item (ie like Hammer's Slammers).
From my other perspective as a player interested in the whole Prime Directive game, I am aware of previous versions and I want to be able to differentiate Traveller driven product from D6 or D20 driven stuff so again the logo is vital, but also secondary - don't get in the way of Prime Directive stuff.
So for me, this
http://www.facebook.com/photo.php?fbid=10150960272313280&set=a.324701708279.150256.231728653279&type=3&theater
Does the job quite well for a front cover although I have to say I think the Traveller logo is a bit too dominant (and thats as a Traveller player looking to move into PD). Its more dominant in some ways than the PD logo. Look at the relative differences in logo size for Hammers Slammers here
http://www.mongoosepublishing.com/rpgs/traveller/hammers-slammers/hammers-slammers.html
Thats more the ratio Id want/expect...