First, excellent work on DOM and IG - there's a huge gap you're filling here.
Essentially, the album thumbnails will need a link to eg : /gallery_page/gallery_album/gallery_item/
This will still load the gallery_album page, mark the gallery_item thumbnail as current and also show the gallery_item details such as the detailed image and caption (and other possible attributes).
If JS is disabled the gallery will still function and be accessible to search engines and with JS all the other fancy UI devices will be available instead.
If you'd like to see this sort of gallery in action, have a look at http://www.theothercompany.co.uk/gallery/.
I am new to silverstripe and it's pretty slow going modifying still, but I'm getting better. Any advice / comments about the above would be greatly appreciated.