I am not entirely sure, but I have enough evidence (based on several sites)
to justify posting it:
Google Images indices have just been updated, which is rare (updates are
several months apart, used to be 5ish). If your site relies on graphics for
some of its traffic, expect an improvement. Having said that, all sites
expand in terms of graphical content, so if you have not added anything
new, you might lag behind.
I know that my labelled photos (yes, _I_ carefully took them) have led to
nearly 200-350 (referall-framed) visits per day. This might be a good SEO
aspect, which provides the users what they want (a relevant photo),
satisfies Google's infinite thirst for knowledge and may bring back to you
some visitors who merely sought a photo, but decided to hang about.