Add a caption to perch:shop image

Hi,

This is probably a stupid question - but I'm trying to work out how to add a caption to a perch shop image.

So we already have:

<perch:shop type="image" id="images"  width="600" height="338" density="2" crop="true" />

I realise you have a lot of pre-defined IDs in there, such as perch:shop id="title" etc., but is it possible to have something like this to go with it?:

<perch:shop id="captiontext" type="text" label="Image Caption" />

I've tried the above, but nothing shows up when I go to edit the product...

Thanks for any help with this!

Alex