Load img if size IS NOT NULL

You could look at refreshing the image using Javascript

This first method wont combat the issue with loading a broken image, however it will update quickly (i've got it set at 500ms - 0.5s), I use a security cam at home and although it does occasionally show a broken image it is only for a very small time:

function refreshImage()
{
    var src = "http://192.168.1.121/snapshot.jpg?" + new Date().getTime();
    var myImage = new Image();

    myImage.src = src;
    myImage.onload = function(){
        document.getElementsByTagName('img')[0].src = src;
    };
}

setInterval( refreshImage, 500 );

Alternatively, you could load the image in with XHR - but you'll probably have to set up a local domain for it to work instead of IP address based. - This will fix the broken image issue - I've added in the Content-Length check but you could probably remove this and it still work fine

function refreshImage()
{
    var src = "http://192.168.1.121/snapshot.jpg?" + new Date().getTime();

    var xhr = new XMLHttpRequest();
    xhr.open('HEAD', src, true);
    xhr.onreadystatechange = function(){
      if ( xhr.readyState == 4 && xhr.status == 200 ) {
         if( xhr.getResponseHeader('Content-Length') > 0 )
         {
            document.getElementsByTagName('img')[0].src = src;
         }
      }
    };
    xhr.send(null);
}


setTimeout( refreshImage, 5000 );