I'd prefer to stick with no additional_tags at all when using the TracTags-Plugin with latest ScreenshotsPlugin. However an empty list is impossible (raises Python error) without explicitly initializing the tags variable as a list variable inside tags.py. A patch to fix this is attached as comment.
valid for:
TracScreenshots? 0.7 (r7486)
TracTags 0.6
test system
Trac 0.12dev-r9115
Python 2.5.2 (r252:60911, Jan 24 2010, 14:53:14) [GCC 4.3.2] - stock Debian package
setuptools 0.6c8 - stock Debian package
pytz 2008c - stock Debian package
SQLite 3.5.9 - stock Debian package
pysqlite 2.4.1 - stock Debian package
Genshi 0.6dev-r1092
Babel 1.0dev-r482
Pygments 0.10 - stock Debian package
jQuery: 1.3.2