Hey there,

I am really sorry for the delay of this reply as it is a public holidays for us here.

Currently I am not sure which Easyblog version you use on your site, but you can try check what Javascript file it render on the composer page, you can view the page source then search for this media/com_easyblog/scripts/composer.

Once you know what JS file it render on the composer page, so try find that JS file and open it, search for this following code :

[gist type="php"]
mediaManager.getVariation(f,"small"),h=mediaManager.getVariation(f,"large")
[/gist]

Replace with

[gist type="php"]
mediaManager.getVariation(f,"small"),h=mediaManager.getVariation(f,"original")
[/gist]

So it will render the original size from that thumbnail pop up section, but this customisation code will not affect with your existing blog post, because those image pop up HTML code already stored in database.

You have to re-upload those image into your existing blog post in order to show the original image pop up.

Note : Make sure you have clear your backend and your browser cache and when you manually access this JS file on the browser .../media/com_easyblog/scripts/composer-xxx-xxx.js , it doesn't show your latest changes, mean somewhere already cached.