You cannot detect directly if the mouse is over a texture, but you can check if it's inside its rectangle. Here's an example:

var normalTexture : Texture2D;
var overTexture : Texture2D;

private var correctedMousePosition : Vector2;

function OnGUI ()
{
    correctedMousePosition = Vector2 (Input.mousePosition.x, (Screen.height-Input.mousePosition.y) );
    if ( Rect(10,10,200,50).Contains(correctedMousePosition) )
    {
    	GUI.DrawTexture( Rect(10,10,200,50) , overTexture);
    	if( Input.GetMouseButtonUp(0) )
    	{
    		Debug.Log("The button has been pressed");
    	}
    }
    else
    {
    	GUI.DrawTexture( Rect(10,10,200,50) , normalTexture);
    }
}

UnityGUI supports mouse rollover by default. There are lots of examples in the docs.

This example will react to rollover and click (from the OnGUI script reference):

function OnGUI () {
    if (GUI.Button (Rect (10,10,150,100), "I am a button")) {
        print ("You clicked the button!");
    }
}

To answer the first part of your question, search the script reference for "mouse" and you should find all kinds of functions for getting the mouse state. Then based on whatever state you're in, your script could use thisTexture.enabled = true and thatTexture.enabled = false to switch visibility of 2 different textures on rollover (for example).