I have done this with jQuery script. Add span tag after main image in product view page & add script in js.
jQuery(".gallery img").click(function(){
var gimgSrc = jQuery(this).attr('src');
var tarr = gimgSrc.split('/');
var file = tarr[tarr.length-1];
var gimgTitle = jQuery(this).attr('title');
// get main img src
setTimeout(function(){
var mainimgSrc = jQuery('.product-image img.product-retina').attr('src');
var tarrrr = mainimgSrc.split('/');
var filem = tarrrr[tarrrr.length-1];
//console.log(file+'------'+filem);
if(file == filem){
jQuery('.product-image span.imgLabel').text(gimgTitle);
}else{
jQuery('.product-image span.imgLabel').text('');
}
}, 600);
// end
})