How to check img src is true using javascript

How to check img src is true using javascript

There are 5 no. of buttons(images). Initially all are off image. when i
press any
button that img src change to on.png. Second time when i press any of
those on or off
button, the pressed button source img change to on.png and other on img
also change to
the off.png. The html code is,
<table cellspacing="0" style="padding:0%; margin:0% auto;">
<tr><td><img id="img1" src="off.png" height="30" width="30"
onclick="off(this.id);" /></td><td>45:78</td></tr>
<tr><td><img id="img2" src="off.png" height="30" width="30"
onclick="off(this.id);" /></td><td>45:78</td></tr>
<tr><td><img id="img3" src="off.png" height="30" width="30"
onclick="off(this.id);" /></td><td>45:78</td></tr>
<tr><td><img id="img4" src="off.png" height="30" width="30"
onclick="off(this.id);" /></td><td>45:78</td></tr>
<tr><td><img id="img5" src="off.png" height="30" width="30"
onclick="off(this.id);" /></td><td>45:78</td></tr>
</table>
The javascript code is,
function off(a)
{
var images = document.getElementsByTagName('img');
for(var i = 0; i < images.length; i++)
{
var img = images[i];
alert(img.src);
if(img.src == 'on.png')
{
img.src = 'off.png';
}
}
document.getElementById(a).src='on.png';
}
The if() condition is not working, Please provide solution and explain why
its not
working. Thank you!