This is however a job of JavaScript to change the attribute of the HTML element upon some event. You can use jQuery for this if you want to shorten the code.
Upon loading the image, you can add this line
if($('img').attr('src') == 'new_delhi.png') {
$('img').attr('alt', 'New Delhi');
} else {
$('img').attr('alt', 'Agra');
}
..you can add your own logic to this too. Otherway of doing this is to create a new child using C# in the serverside page and passing it down to the user using Ajax request. But that will be too broad. You can simply just change the source of the image and upon changing the source, change the alt attribute of the image too.