Actually… why not making the transclusion sign ! valid both outside and inside the link? i.e.,  would be equivalent to [alt](!target-image.jpg). (By following this logic, we could even turn alt text into a visible caption in a simple and consistent manner, i.e. [!caption](!target-image.jpg).)
This would retain compatibility with current markup, and would allow a simplified image link markup that is both simple and unambiguous: [alt](!cats-big.jpg)(cats-small.jpg).