<object data="foobar.png" width=200 height=200> <img src="test.png" alt="Just testing."> </object> 
Here foobar.png is the main image, test.png is the inverse image. According to the semantics of the object element, the content of the element (here the img element) should be displayed if and only if the primary data (specified by the data attribute) cannot be used.
Despite the fact that browsers had terrible errors in object implementations last year, this simple method works in modern versions of IE, Firefox, and Chrome.
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