Hello @pranalipatel ,
Thanks for your answer.
I would display the other images as thumbnails but directly in the lightbox, near the comments. So there would be a main image in the lightbox , and near the comments there would be the other thumbnails.
By the way, I have another problem : the delete button is not displayed in the rtmedia uploader, so I can't delete images before upload. I have downloaded the new version of rtmedia but I still have this problem (below an example):