You have to put the asset in a folder called "Resources". Then it's a matter of using Resources.Load.
Texture2D myTexture;
void Start()
{
// Given you have placed Images/myImage.psd in a folder
// called Resources:
myTexture = Resources.Load("Images/myImage") as Texture2D;
}
void OnGUI()
{
GUI.DrawTexture(new Rect(0, 0, 50, 50), myTexture);
}