You could publish the GUIContent it would use. This allow you to set text, an image and a tooltip. If you don't want to use text or tooltip, just leave them out.

var buttonContent : GUIContent;

function OnGUI() {
    if (GUILayout.Button(buttonContent)) {
        // Handle the press
    }
}

If you rather want to make a button with a custom style, you could either make a new style in a GUISkin or define the style directly on the script.

var buttonStyle : GUIStyle;
var buttonContent : GUIContent;

function OnGUI() {
    if (GUILayout.Button(buttonContent, buttonStyle)) {
        // Handle the press
    }
}

Or with a custom skin/style:

var customSkin : GUISkin;
var buttonStyle : String = "button";
var buttonContent : GUIContent;

function OnGUI() {
    GUI.skin = customSkin;
    if (GUILayout.Button(buttonContent, buttonStyle)) {
        // Handle the press
    }
}

  var btnTexture : Texture;

  function OnGUI() 
  {
      if (GUI.Button(Rect(10,10,50,50), btnTexture))
      {
          Debug.Log("Clicked the button with an image");
      }
  }