I just gotta rub it in now. ^^
https://www.dropbox.com/s/c3txeo4dlkruev6/LolClient%202016-01-23%2016-41-30-26.jpg?dl=0 https://www.dropbox.com/s/lf0jt4vhs580mso/League%20of%20Legends%202016-01-23%2020-42-48-85.jpg?dl=0
Nah, it's actually quite simple - you replace the files:
Graves_0.jpg
Graves_Splash_0.jpg
Graves_Square_0.png
Located in (current patch - the releases number will change):
[X]:\League of Legends\RADS\projects\lol_air_client\releases\0.0.1.195\deploy\assets\images\champions
X = Your local install drive. And without the [].
You will ofc. need the files you want to put into the client, and probably have to edit them (like I did with the Irelia splash above) to make them fit, so some image editing software is required.
To change the load splash, you will need additional 3rd party software and is quite a bit more tricky, so don't mess with that unless you know what you're doing.
I did this change with the King of Clubs Mordekaiser skin too...well, to be accurate, I just reverted back to the original one; to the one with the "illegal" cross on the chair.
http://3.bp.blogspot.com/-zVsenVOKghE/VWVbmrEqF2I/AAAAAAAAEhU/EVT8UHoTcCY/s1600/lol.congdonggame.net_1.jpg
As it apparently was offensive (BS!), that was all the reason I needed to put it back into the client.

I prefer to exchange information like this, and I currently need the information on how to create .swf compressed archives. Are they made using some Adobe software?
I want to mod the summoner icons, and while I have found the files that need editing, I can't currently recreate them; only open them and extract files, but not replace or insert anything.
I NEED this icon:
Doesn't matter if it only is I who see me having it.
http://vignette2.wikia.nocookie.net/leagueoflegends/images/1/10/ProfileIcon_Frostguard.png/revision/latest?cb=20130728183254
...but I signed up to this game less than a month after the event had ended. :(
Tried bribing Rito support with a 1000 RP to give it to me, but they said "doing underhand deals is against our company policy". :/
The offer of 1000 RP still stands btw, if any Rioter sees this. Can go up to 1500 RP if required. ;)