I have struggeled with the same, and ended up modifying the Image.as file - but I am not 100% happy with the result. For the time being, it looks like this: lystfotograf dot net
By use of the "h" variable i put the caption at the bottom of the image, and, in additon, adds some pixels downwards to put it below the image. This looks OK if the image height is less than the stage height, but if it is the same or higher (using aspect ratio to check this) I move it upwards to avoid placing it in the "black hole" below the stage. The caption will then be placed over the image.
Currently I am struggling with finding out how I can refer to the bottom of the stage, independent of image height. I will also try to add a space below the image stage and reserve it for caption use.
The decription above may look a bit messy, but that could be due to lack of the total understanding how the viewer is working :/ I am also not an .as programmer and by no means familiar with Flash.