How to display attached images as links in phpBB3
In phpBB3 attachments are enabled by default. Most image formats (e.g JPG, PNG, GIF, etc.) are also allowed as attachments. This means that unless you change the permissions of particular users or user groups, registered users can attach images in posts (you can additionally enable image attachments in private messages). The attached image will be displayed in the post, under the text of the message.
If you find it more convenient, however, you can change the settings in order for the attached images to be displayed only as links. When someone wants to view or download the image, they can click on the link. To make this change, log in to the administration control panel of your phpBB3 board, click on the Posting tab, and then click on the Attachment settings link on the left. On the Attachment settings page scroll down to the section labeled Image category settings, and mark the button No for the option Display images inline. Don't forget to click on the Submit button to save the change. Now all attached images will appear as links.
Otherwise, if you haven't changed this option, and attached images don't appear as links but inline, each registered user can set his/her attached images to be displayed as links. Registered users can do this from the user control panel: Board preferences>Edit display options>Display images within posts to No. Of course, if you changed the setting globally from the administration control panel, this option won't matter and all attached images will be displayed as links.
From the Manage extension groups link of the Posting tab page you can add more image formats to the allowed ones.