Transferring all image attaches into img

[Inna]

Hi, before saying anything, I know that many tables are related to the posts and attaches, so large modifications should be done in this case:
There are many images which have been attached and the registered user can see thumbnails and also click on the pic to see it in full size, but unregistered users (guests) can only see thumbnails.
Image extensions I've defined are jpg jpeg png gif.
So, there's no problem since here, yeah?
But when the attachment is put into [img] tag, the guest can see that picture since it's a normal picture and not an attachment, like the picture below which is taken in the guest mode:

I want all attachment images png gif jpeg jpg which are in posts, deliver an [img] tag in order to be shown for guests.
I mean thumbnail is at the end of the post, and this img tag be included above this thubmnail.
Since posts and attachments are a lot, editing personally is not available.
And also after that, all images which are going to be attached, should be delivered an img tag for being shown to guests!
Is there any product which can do this?
What about the way of doing it?
Any sql running in phpmyadmin?