When you click a thumbnail, it takes forever to load the image.

“About : at http://jenny-long.com/wp/newwork/
(It uses twentyten child theme and NextCellent Gallery plugin (it’s up to date. And WordPress 4.5.2 version.)
When I click an image from a thumbnail page, loading an image takes forever especially toward the bottom of the thumbnail list.

There are 39 thumbnails on a page. Is that too many? If so, what’s the optimum number of images on a thumbnail page?

Should I optimize each image? What is the suggested size of an image? Is the max size of an image — say 1 MB for example? I have some image as large as 2.7 MB. Is that too big? Is that why the image load is very slow? (sometimes it never finish “loading”.)

Yes, it sometimes takes forever for a picture to show up on the screen –if you click on a thumbnail toward the bottom end of the thumbnail list (not Slideshow page, but “Picture list” thumbnail page)…never finish “loading”.

How do I remedy this? Is the image too big a file? What size is too big?
Thank you for your help.”

https://wordpress.org/plugins/nextcellent-gallery-nextgen-legacy/