Alternative Image - Original Image Is Not Found On The Server

I want to acheive the following functionality using JAVASCRIPT -:

To be able to specify alternate URL incase the original image is not found on the server.

In HTML there is functionality for displaying alternate text.

But I want some functionality for alternate image.

I saw a solution wherein I could iterate over all the images in the html page and then verify whether img.completed==null. But surprisingly eventhough the image is not found , the value of completed is not null. So I felt that using the completed atrribute was not the right way.

I either need to verify whether the image exists on the server using JAVASCRIPT or do some other thing with some other attribute which might not have come to my attention till now.