Clicking a thumbnail to change a larger image

Question

I've tried to change a main image when clicking on a thumbnail, but I can't seem to have two different instances of the code together; only one will work at a time. Can someone tell me what I'm doing wrong, please?

My Javascript -

function changeImage(img) {
document.getElementById("img").src = img.src.replace("_t", "_b");
}

function changeImage(img) {
document.getElementById("img1").src = img.src.replace("_t", "_b");
}

My HTML -

<img src="images/AGM/events_t.jpg" 
onclick='changeImage(this);' 
onmouseout="this.style.opacity=1;this.filters.alpha.opacity=100"          
onmouseover="this.style.opacity=0.5;this.filters.alpha.opacity=0.5"> 

<img src="images/AGM/events1_t.jpg" 
onclick='changeImage(this);' 
onmouseout="this.style.opacity=1;this.filters.alpha.opacity=100"    
onmouseover="this.style.opacity=0.5;this.filters.alpha.opacity=0.5"> 

<img id="img" src="images/AGM/events_b.jpg" width="650">



<img src="images/BLACSBF/events_t.jpg" 
onclick='changeImage(this);' 
onmouseout="this.style.opacity=1;this.filters.alpha.opacity=100"    
onmouseover="this.style.opacity=0.5;this.filters.alpha.opacity=0.5"> 

<img src="images/BLACSBF/events1_t.jpg" 
onclick='changeImage(this);' 
onmouseout="this.style.opacity=1;this.filters.alpha.opacity=100"    
onmouseover="this.style.opacity=0.5;this.filters.alpha.opacity=0.5">

<img id="img1" src="images/BLACSBF/events_b.jpg" width="650">

Answer 1

Create one changeImage function and pass two parameters, the image and the target div.

function changeImage(img,target) {
    document.getElementById(target).src = img.src.replace("_t", "_b");
}

Then call it like onclick='changeImage(this,'img1');' or onclick='changeImage(this,'img');'