Currently phpBB 3.0.9, and any previous versions. Are only adding the <img> tag for smileys, with no CSS usages.

Most browsers tend to display the picture a bit higher than the text. And as phpbb shows them as simple images, they are a bit highter too..

I would advice using " vertical-align: text-bottom; " on the smileys, so that the smiley will display on the same line as the text.

I hope i explainded well... If not, have a look on the two attachments.

Notes:
In the attached image, the first two lines on top are using the CSS property.
The second two images are the default of phpbb.

http://area51.phpbb.com/phpBB/viewtopic.php?f=4&t=41831