How to fix in magento 2 404 images? When magento root in PUB folder?

You might be missing .htaccess in your /pub/media/ folder.

Since you made changes to Apache, you might have disabled the Options FollowSymLinks setting (since you mentioned that you increased security).

So you might have to change

Options +FollowSymLinks

to

Options +SymLinksIfOwnerMatch

in .htaccess in the following locations:

{Your root Magento directory}

/pub/

/pub/media/