[tubes]

Not a good idea. Browsers that do not support imgset would show all three images using that syntax. A new element name like "<set>" would be ignored by them.

[Kilimanjaro]

CSS to the rescue

    imgset img{display:none;}
    imgset img:first-child{display:block;}

Same with modern HTML5 tags, modernizer or any other tool would handle that for you.

[user24]

Not really, what about browsers that don't support first-child? Backwards compatibility is important.