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Hello,

I know SMF and also Elkarte can create thumbnails in PNG format. I wonder what is the advantage using a thumbnail?
I immagine the png format was choosed to maintain transparency on thumbnails but PNG files are lot more heavy than normal jpg (in my case from 20 to 50 Kb for a 150 px width image)

p.s. is there a quick way to remove them?

Disable it from the admin panel: admin > forum > attachments and avatars > attachment settings => "Save thumbnails as PNG"

I disabled the creation of thumbnails but they still exist. I can also browse them from index.php?action=admin;area=manageattachments;sa=browse;thumbs

Is there a valid reason to use them?

If you want to show thumbnails of attachments below the post, and not have to download the entire full sized image, you need thumbnails. 

Today if you disable thumbnails and still have "show thumbnail under the post", you will get (I think) the full sized image under the post (and of course all the data of the full sized image must be sent).  Sure you could force the thumbnail with CSS on that full sized image but you would still be sending the data of the entire full sized image.

If you don't want to display attachment thumbnails under the post, then you could turn off thumbnail creation and turn off show thumbnail under the post.

So their use is  to show the image thumbnail preview under the post, and not have massive page weight.  As you noted PNG's will be better for graphic thumbnails, and jpg for image ones, so if you are mostly doing images, don't select create them as PNG.