Here is the picture of the basic I've made.

I thought that it was too plain and would really appreciate some useful advice and ideas that can make it come alive.

First of all, the colours. They're too bright and saturated, esp given the tired look on Athrun's face.

It seems that you're trying to create a scene to go with it, maybe add more details, like where is he sitting? on a hill? on a beach/ocean-side? underneath a tree? Also, think about the mood you want the finished product to convey. Is this supposed to be a happy wall where he's just taking a break from working, or is it supposed to be more serious, like he's deep in thought? That will affect your colour scheme as well as the elements you put in.

Instead of using a flat colour for the sky, use a gradient, and also use more blue-ish colours instead of cyan. Then you can either brush in some clouds, or incorporate stock, or use cloud brushes.

The grass. The perspective is wrong. Don't use the same size brush for the whole grass. Esp from that angle, the blades near the horizon would have to be smaller than those near the bottom (ie closer to the viewer). Also, vary the colours a bit, but don't use such saturated colours (esp that highlighter green, it makes the grass look unnatural). At this point, the grass looks rather thin, maybe use smaller brushes at a higher density to make it look lusher. Also, when brushing the grass, start from the back (ie near the horizon) and work your way down. That way, the roots of the grass near the back will be covered up by the blades of the grass at the front. Ah, I'm sure there are tonnes of grass tutorials out there that can explain things much clearer than I can...

Also, the dirt needs to be less bright as well, try for a darker brown instead of an orangy brown. And instead of using a straight horizon ('cause that's kinda boring), how about making it slightly hilly.

The placement of Athrun looks good though. Keep working at it! hope that helps

clour dun really suit scan. maybe make it something like in the night sort of thing, it will look better. for the sky, you can find stocks but modify it...! ^^ then maybe put in like moon and stuff like that. i dun think you need the soil hahax..! ^^ use brushes if you have to make it look moreattractive, hmm.. ic athrun is sort of sweating right, you can try to make the effect that maybe it's raining. yup, put rain drops[brushes]. it would look nicer..! ^^ scan and extraction good..! ^^ good luck for the outcome..! ^^

Thanks for ya opinions, I had a night version in mind originally, and blue patch as a lake or something like that, but I have difficulty making a realistic reflection. Any ideas?

hmm, have you tried stocks? or realistic water reflection tutorials?
hmm...you can try this one, http://www.rnel.net/tutorial/Photoshop/4079

yup..! ^^

above is a night version I've tried, using a night sky this time. what do you think?

looks much nicer, but the grass seems too dak, making it like a whole black clum..! ^^ try using a dark green..! ^^

The grass is a bit too dark, I agree with norine. Use a dark green, or check out Nat's grass tutorial.
The stars look ok, though I think there might be too many, or maybe they're just too bright. I would also try putting some "fireflies" coming out of the grass.

I have nothing else to say, they all says what I wanted to say :-(. By the way, thanks to Chloe-chan and norine07, the tutorials were really good.

Wow! I like how you extracted the moon here and the stars look pretty. Though maybe the grass needs to be a little. I liked how Chole-chan showed a link of some tutorial because they can be helpful. Maybe try making Athrun blend with the grass and add some fireflies near the grass (To make it a lovely resting spot ) It's simple just use a brush and use any size you want, but make the opacity lighter if the fireflies are at the distant.

Though I must admit that youe extraction is good.

Thank you so much, I've been away from camp for some days now and haven't really had time to reply or work on your suggestions which I really appreciate them.

I think the wall has improved a lot. The stars and moon do look nice, but there are a bit too much, personally. Try adding variation of star clusters instead of have a whole bunch of stars all over. The moon seems a bit large too; it competes for attention, which I do not think is your intention. I agree that the grass is too dark, it becomes more like a patch of black. Now one of the biggest problems I noticed is the tree. The edges are way to jagged and could be smoothed out at some parts. The edges seem a bit too sharp also, try adding a slight blur to blend it into the background a bit more. Also, try adding some highlights on the tree to add some dimension, so it looks like the moon is casting light onto the tree. I hope you consider these suggestions and I look foward to your revisions!
Good work so far!