tizz

@samster19, at http://newenglandphotography.net/ I see what you requested for. Clear your cache.

@denisephilipbar, the issues are similar but they could not have the same cause, so everyone gets the most appropriate attention in their own topic.
To both I may further suggest to try another lightbox effect among those available, but please @denisephilipbar open your own topic, as per the forum rules.

@jakob1975 I don’t think your two installations are perfectly identical. Probably in your production site you haven’t tested in the right way, e.g. in the page that works I don’t see the SEO plugin activated as in the other, and in the page that doesn’t work also social icons and menu and title icons doesn’t work, so it seems that some stylesheets and js scripts aren’t loaded.
You have to deactivate all of your other plugins at once, clear browser cache completely, update the page. Also, I see that you have a cache plugin, so before this operation you have to empty all your site caches via that plugin.
With any plugins activated but NextGEN, caches emptied (site and browser), update the page and it should works. If not, deactivate your current theme too and activate one of WordPress default theme, and in this basic situation it should works.
After this test you’ll know for sure if something is in conflict, or if you have to investigate at another level.

Gallery Settings > NextGEN Basic Slideshow > Enable flash slideshow > No

@northernmonkeyz, I don’t know what you mean when you talk about “posting to category method”, but installing NextGEN doesn’t involve your posts in any way.
Surely if you want those 60+ images displayed with NextGEN you will need to upload them as NextGEN galleries, and you can display them where you want it, those same posts or whatever post or page.

@alejandromb, there aren’t any NextGEN files loaded in that page, so ask to the vendor/author of your premium framework.

@cagatay Belgen Yes I know that is not ideal, maybe a function in functions.php it would be better.
Sorry I’m not a coder but I have modified those “callings” in home.php in that way and it works well, so I have shared my fix for OP @cyrilb245, or anyone interested in.
Anyway, sizes are different for some categories maybe, not for every image. So you can have some categories in home page (the “problem” is only in home page, not for sidebar and widget), but I don’t think that you have more than 4-5 categories (do you have?) – beyond the fact that all categories can have featured image of the same size, it’s even better aesthetically.
Also, the missing size is only for the first image of the category, the bigger one, not for the other three images in the category.
A sample of what I have done:

<?php if ( in_category (123) || in_category (45) ) { ?>
<img src="<?php echo leaf_get_post_image( null,null,true,null, 'large' ); ?>" width="300" height="120" alt="<?php the_title(); ?>" class="attachment-post-thumbnail wp-post-image">
<?php } elseif ( in_category (67) ) { ?>
<img src="<?php echo leaf_get_post_image( null,null,true,null, 'medium' ); ?>" width="217" height="200"  alt="<?php the_title(); ?>" class="attachment-post-thumbnail wp-post-image">
<?php } elseif ( in_category (89) || in_category (101)  ) { ?>
<img src="<?php echo leaf_get_post_image( null,null,true,null, 'medium' ); ?>" width="200" height="300"  alt="<?php the_title(); ?>" class="attachment-post-thumbnail wp-post-image">
<?php } ?>

I don’t know if there’s a better way to write it.