Custom attributes
To add custom attributes such as e.g. alt add to <a> tags attributes with 'data-' prefix:
<a data-fslightbox="gallery" data-alt="Example image description" href="img.jpg">
Image
</a>
Image
</a>
Lightbox will add alt attribute to source:
<img src="img.jpg" alt="Example image description">
You can add any attribute you want:
<a data-fslightbox="gallery" data-alt="Example image description" data-foo="bar" href="img.jpg">
Image
</a>
<!-- Rendered source -->
<img src="img.jpg" alt="Example image description" foo="bar">
Image
</a>
<!-- Rendered source -->
<img src="img.jpg" alt="Example image description" foo="bar">