I have to say, those look really spiffy. I too am a big fan of the second-to-last one. The way planet glows in the giant sun looks very, very nice. The first and third ones remind me a lot of X3, with the distant suns and close planets. I'm a big fan of still-shots for title screens; those look very professional.
If you're looking for ideas, one thing I'd suggest is a shot similar to the last picture, only a close-up looking down the side of a Battleship or Station.
Kinda like this
, only not so dark, and maybe a little further away. It may or may not work, but if placed in something like a nebula cloud I think it could make a nice still-shot.
Also, I'm just curious, would you be interested at all in making a set of 3 for the project I work on, 88 Flak? The only things I'm using for now are just simple base THNs, as I never quite figured them out (using bases is kinda neat though, because FL remembers the last ship you used and places it on the landing pad). I'd love to have something like those, and if you're interested, drop me a line via E-Mail (it's in my profile)