https://www.icyphoenix.com/viewtopic.php?f=26&t=3055&p=22549#p22549
-----------------------------------
Artie
Sat 22 Dec, 2007 23:21

Re: Image Border
-----------------------------------
For a standard phpBB (Non Icy) board ... 

OPEN [b]templates/YOUR_TEMPLATE/bbcode.tpl[/b]

FIND

[codeblock]<!-- BEGIN fullalbumimg --><a href="album_showpage.php?pic_id={IMG_NUM}" target="_blank" class="postlink"><img src="album_picm.php?pic_id={IMG_NUM}" alt="Image {IMG_NUM}" title="Image {IMG_NUM}" border="0" /></a><!-- END fullalbumimg -->
<!-- BEGIN albumimg --><a href="album_showpage.php?pic_id={IMG_NUM}" target="_blank" class="postlink"><img src="album_thumbnail.php?pic_id={IMG_NUM}" alt="Image {IMG_NUM}" title="Image {IMG_NUM}" border="0" /></a><!-- END albumimg -->
<!-- BEGIN albumimgl --><a href="album_showpage.php?pic_id={IMG_NUM}" target="_blank" class="postlink"><img src="album_thumbnail.php?pic_id={IMG_NUM}" border="0" align="left" alt="Image {IMG_NUM}" title="Image {IMG_NUM}" /></a><!-- END albumimgl -->
<!-- BEGIN albumimgr --><a href="album_showpage.php?pic_id={IMG_NUM}" target="_blank" class="postlink"><img src="album_thumbnail.php?pic_id={IMG_NUM}" border="0" align="right" alt="Image {IMG_NUM}" title="Image {IMG_NUM}" /></a><!-- END albumimgr -->
<!-- BEGIN albumimgc --><center><a href="album_showpage.php?pic_id={IMG_NUM}" target="_blank" class="postlink"><img src="album_thumbnail.php?pic_id={IMG_NUM}" border="0" align="center" alt="Image {IMG_NUM}" title="Image {IMG_NUM}" /></a></center><!-- END albumimgc -->
[/codeblock]
REPLACE (where desired)
[codeblock]border="0"[/codeblock]
WITH
[codeblock]border="1"[/codeblock]

or use DR's suggestion and replace [b]border="0"[/b] with the style attributes