Level unlock system

This is my main menu:
45106-unlock.jpg

I have placed a picture with a collider in front of the level 2 button. When I finish level 1 I want that picture to get destroyed forever. (I think that would be the easiest way.) In other words, how can i make a level unlock system.
And yes I HAVE tried this method: Loading...
And i can’t make it work.
Appreciate answers!

You can use PlayerPrefs and save the number of unlocked levels (as an int). The you launch the game you can check that number and if the button level N is superior to the unlocked count, it is locked.