This tutorial is for the Seasons theme. It could be useful for other themes.
I wanted to use a lightweight jQuery lightbox to view larger images on the Seasons theme and came across Featherlight - http://noelboss.github.io/featherlight/ - which looked great.
Using the tips on the Adding Lightbox to Omeka page (https://omeka.org/codex/Adding_LightBox_to_Omeka) I have modified Seasons so that the large image on the item show page, when clicked, shows the full size image in the Featherlight lightbox.
Open seasons/items/show.php in a text editor.
Add this to the very top of the file (before <?php etc)
Look for the code (originally line 8):
<?php echo files_for_item(array('imageSize' => 'fullsize')); ?>
And change it to
<?php echo files_for_item(array('imageSize' => 'fullsize', 'linkAttributes' => array('data-featherlight' => 'image'))); ?>
Save/upload the file replacing the old version (which you did back up, didn't you?)
Featherlight should now work. Visit an item page which displays an image and click it - a much larger version should be displayed in a nice responsive lightbox.
Hope this helps some folks.