How to change block texture on each side?

The title says it all
How to change block texture on each side?
I tried

Code:

    public int getBlockTextureFromSide(int i)
    {
        if(i == 1 || i==0) //1 is top, 0 is bottom
        {
        return blockIndexInTexture = ModLoader.addOverride("/terrain.png", "/WEmod/travelb/top.png");
        }
        else{return blockIndexInTexture;}
    }

But it doesnt seem to work, recompile works, reobf works, just the image doesnt work, the top and bottom of the block seems to be the purple-ish block texture in terrain.png and the command prompt keep saying
Overriding /terrain.png @ (this is number from 50+ to 255).....
when it hits 255, the game crash, which is normal for this error
Anyone know how other mods change block texture on each side?

Kaekae has a function for this. Take a look.

Ziddia wrote:Kaekae has a function for this. Take a look.

Where would I find this function? I too would like to do this

Please move to the new forum, http://www.zidmc.info