Activate GuiTexture When Clicking on BoxCollider c#

 public Camera theCamera; //set this to your camera in the inspector
 public Texture2D YourImage; //set this to your image you want to show
 private bool shouldIEnableTheGUITexture = false; //to determine if we need to show the image or not

 void Update(){
     //if there has been a mouse click
     if(Input.GetMouseButtonDown(0)){
         if(Physics.Raycast(theCamera.ScreenPointToRay(Input.mousePosition), out hit)){
         //a collider was hit, if the name of that collider is "blah", the "blah" button was pressed!
             if(hit.collider.name.Contains("THE_NAME_OF_YOUR_GAME_OBJECT_WITH_THE_COLLIDER"){
                 //enable gui texture
                 shouldIEnableTheGUITexture = true;
             }
         }
     }
 }

 void OnGUI(){
     if(shouldIEnableTheGUITexture){
         GUI.drawTexture(new Rect(0,0,100,100), YourImage);
     }
 }

Only three things you need to do. Add your main camera to the inspector variable named "theCamera", change "THE_NAME_OF_YOUR_GAME_OBJECT_WITH_THE_COLLIDER" to the name of your gameobject that has the collider, and add your image in the inspector. Attach this script to any gameobject.