I spent some time today figuring out how to display the custom fields with the images - you need to edit the template(s) being used. For example, in the

wp-content/plugins/nextgen-gallery/products/photocrati_nextgen/modules/nextgen_basic_gallery/templates/thumbnails/index.php

template, I changed the display of the thumbnail to include the source field by adding this:

<br/>Source: <?php echo nggcf_get_field($image->pid, "Source");

to the existing title attribute. I expect this will be transferable to other templates and fields.