HOW DO I DISABLE BUTTONS ON image FADE IN?

I have four buttons on a website - on press I want each button to trigger a fade in for an image…When a user hits on diff button, the the previous image should fade out and a new image fade in…
The script I am using doesnt work all the time…sometimes when i press a button…the i press a diff button…two images will fade in overlapping eachother
Can someone please take a look at my file and tell me how to make this concept work with no errors???
thanks