Get texture from gui to gameobject

Hello I have some problem with GUI. It is possible to change texture of selected game object from GUILable or from any GUIButton . I am using with OnMouseDown function for this but this function is not usable for GUI content so anybody know that how i can.
I used guiTexture.texture=GameObjectName;


Texture btn_texture; // Texture used in the label or button or whatever

void OnMouseDown()