A module's templates decide how its pages look: item_detail.tpl for an item's page, item_list.tpl for lists, and so on. Change them without touching the module, so updates don't undo your work:
(module)_(template). Best for larger changes, and kept with your skin.Either way, work in a cloned skin. See Cloning a Skin.
Copy the module's template into your skin's folder, named after the module:
modules/jrGallery/templates/item_detail.tpl -> skins/xxMySkin/jrGallery_item_detail.tpl
Jamroom now uses your copy for every gallery image page. Change it as you need. A quick test: add some text at the top, and check it shows. If it doesn't, reset the caches, or turn on Run in Developer Mode in Developer Tools.
To change one list only, not the module's template everywhere, give that {jrCore_list} its own template instead. See Changing How a List Looks.
In jrGallery_item_detail.tpl, wrap the image in a link to the large version, with data-lightbox, so clicking it opens the lightbox Jamroom already includes:
{jrCore_module_url module="jrGallery" assign="murl"}
<a href="{$jamroom_url}/{$murl}/image/gallery_image/{$item._item_id}/1280" data-lightbox="images" title="{$item.gallery_title|jrCore_entity_string}">
{jrImage_display module="jrGallery" type="gallery_image" item_id=$item._item_id size="1280" class="img_scale" alt=$item.gallery_alt_text width=false height=false}
</a>When the module is updated, its template may change too. Compare your copy with the new one: the module's Templates tab has a Compare button. See Keeping a Cloned Skin Up to Date.