It appears that you're going for a scenic scene here, so what I would do is get rid of the 'Ghost Lacus'
since she really doesn't fit. Also, drop the shadow on the other Lacus since it makes her stand out too
much.
For the BG, I would consider getting some decent clouds brushes. Try looking at deviantArt's Resources section.
They have some really good ones. I would also do more than one layer for the grass, using two-three colors of green.
This'll add some depth to it. And if you're going to have some leaves falling, why not add a source to show
where they are falling from? Could be as much as a few branches with falling leaves above Lacus.
As for the scan, as was stated, it appears a bit pixelated. Were you possibly using a smaller scan and enlarging it? The
extraction also isn't that great, so I would work on that too, maybe try using the Pen Tool and getting really
close to the border.
You also need to make your signature smaller because it really does take away from the wall. People focus more on that
then on the wall itself. Normally, I use about a size 14 font, but it depends on what font you are using, so you may
need to go smaller or even a bit bigger. You need to sign your walls, but you don't want it to be so big that
it's obtrusive to the wall.
With some work and practice, this could turn into a really great wall. :)