You should get a better answer in this forum
The answer to your problem is to use the object.clip method.
(I don't know if this is how these particular adverts do it, but this is what i'd use)
Clip isn't really a method, it's actually a css attribute (technically not a filter, but it acts like one)
Hope this helps
<img src="bla" width="300" height="400"
style="clip:rect(0 300 200 0)"
onmouseover="this.style.clip='rect(0 300 400 0)';"
onmouseout ="this.style.clip='rect(0 300 200 0)';">