To make things even easier you could use the animation engine LeanTween and accomplish this with very little code:

public var grumpy:Texture2D;
private var grumpyRect:LTRect;

function Start () {
    grumpyRect = new LTRect( -grumpy.width, 0.5*Screen.height - grumpy.height/2.0, grumpy.width, grumpy.height ); 

    // Slide in
    LeanTween.move(grumpyRect, new Vector2(0.5*Screen.width - grumpy.width/2.0, grumpyRect.rect.y ), 1.0, {"ease":LeanTween.easeOutQuad});
}

function OnGUI(){
    GUI.DrawTexture( grumpyRect.rect, grumpy);
}

There are more advanced examples here.

Here's a Unity thread about Lerping a GUI element .. compliments of Google: Animating GUI movement?