There are more untagged hidden characters in this and I'm too lazy to search for them and research their names. There's one in the lake and the one getting burned by the dragon is too small for me to identify.

there's a sign which hv names of characters

1st row
Vegeta · Trunks · Nagato
2nd row
Konata · Tsukasa · (couldn't read the 3rd kanji, the first 2 says Sekai, but it could be other reading)
3rd row
Bubble Slime · Slime
4th row
Yokohama Tire · Chimera · RODO (could be Lord)
last
抵大乙 (i hv no idea what this is)

since the obvious character clearly visible aren't in it, could be the names of hidden chars

Vegeta: Trees on the left below the mountain
SSJ Son Goku (or maybe Trunks?): To the right, Half mountains half lake
Nagato: fighting the dragon on the upper-right (ahh, i see now)
Konata: Ahoge only
Tsukasa: Left clouds

Can't find the Yokohama Tire (logo?)

Don't know the others that you mentioned and still can't figure out what/who is that on the center of the lake.

Confirmed, that is SSJ Trunks. His hair is slightly parted in the middle.

phew, a lot to say...

1. I see that you don't put notes so others can have the joy of searching as well
2. is there any significance of the tree larger than mountains on the back there?
3. there has to be a larger pic out there, this one is desktop resolution...

ouch, sorry, huge mistake, the 4th row is not RODO, its ROTO (Lotto)

世界樹 solves two mysteries in one. I think at the bottom of the sign it says 拡大乙, which could be Hiroshi Ootsu if it weren't in the wrong order. Roto might be related to http://en.wikipedia.org/wiki/Emblem_of_Roto.

There's a huge face on the right, but I'm not sure who it is.

and where is churuya?

well, she'll be small, but if you managed to miss that, won't you have problems with the "more hidden" chars?

LaC said:
There's a huge face on the right, but I'm not sure who it is.

Looks to me a bit like a Yukkuri face.

I can't seem to see where Trunks is in the lake =\.

antx said:
2. is there any significance of the tree larger than mountains on the back there?

That's the World Tree from Tales of Symphonia

Yuki Nagato vs Red Eyes Black Dragon?

I'm pretty late to this, but I thought I'd point out Bubble Slime being a leaf in the middle of the screen, and what I think to be Slime, which is that darker area of grass, the oddly distorted trees and the, what looks to be, eyes of the slime on the lake.