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