Can't display PNG texture within GUI calls

While this question has been brought up numerous times, I've yet to come across a viable answer.

I have 4 PNGs to display in my GUI at the moment. Within our single script that does anything to alter the GUI, we have

 var spikeicon : Texture2D;
 var slamicon : Texture2D;
 var mineicon : Texture2D;
 var heaticon : Texture2D;
 
 ...
 
 function OnGUI () {
 ...
 GUI.Box(Rect(Screen.width/2 - 125, Screen.height - 50, 64, 64), 
     GUIContent(spikeCD, spikeicon));
 GUI.Box(Rect(Screen.width/2 + 5, Screen.height - 50, 64, 64), 
     GUIContent(slamCD, slamicon));
 GUI.Box(Rect(Screen.width/2 + 135, Screen.height - 50, 64, 64), 
     GUIContent(mineCD, mineicon));
 GUI.Box(Rect(Screen.width/2 + 250, Screen.height - 50, 64, 64), 
     GUIContent(heatCD, heaticon));
 GUI.DrawTexture(Rect(150, 150, 128, 128), spikeicon)
 }

Screenshot shows that I've placed the textures in the Inspector, as well as where they are currently located in Assets.

Now, I've tried to simply do a GUI.DrawTexture call just to test, and it gave a compile error that the variables had not been assigned, despite them showing up in the Inspector. Furthermore, I've done GUI.Button and GUI.Label, as well as a straight GUI.Box call w/o text so there is no GUIContent call; nothing. I've also tried larger box sizes, considering that the textures are 128x128, but even with a 200x200 box they did not display. They're not mentioned in Start() at all, but I believe they don't have to be if they're placed within the Inspector. If this is not the case, what would be the correct way to go about it? Otherwise, what would I need to do to display the icons properly?