Some of the files are directly loaded from game.exe and some other are loaded from login3.wld
Hex edit login3.wld and look for .SMOD files in the code, then replace them for other .smods from the game (you can see how smods looks like with shaiya studio).
The green moon is:
Login_effect_back01a.smod
Login_effect_back01a.dds <<<< just edit this texture
That energy like effect from corner to corner is from
Logineffect_smoke01a.dds
The main structure si composed of several diferent objects.
You can replace the building with some other object, like an ancient stone or something.
Keep in mind that the new object will still be 'animated' (rotation).
It's mostly trial and error to find something that looks good.
And yes, there are a few post about how scenes are composed, and useful info by Castor, in the forums.
I made this little example from what I remember:
1) Data\Entity\Texture\Logineffect_smoke01a.dds
2) Data\Entity\Building\login_outsideframe01a.SMOD (that rotating ring)
3) Data\Entity\Building\Login_hugerock01a.SMOD
4) Data\Entity\Texture\Login_effect_back01a.dds
5) Data\Entity\Tree\login_floatrock01c_front.SMOD
Other files involved:
Data\Entity\Tree\Login_stonedapple.SMOD (little stones floating)
Data\Entity\Texture\login_effect_acrossdown.dds (big ring in the base of the main structure)
Keep in mind that some objects have their origin point moved from their center, so when you replace them, they won't appear in the same position.
Very Important: Backup up your game.exe en login3.wld file often, 'cause its easy to mess up, and it will cause your client to crash.