I assume your knowledge on SEO is either non-existent or you haven't checked the HTML the K2 outputs...
K2 uses the "alt" and "title" attributes in images and wrapping links to either include image captions or if no image captions are entered by the user, then just the title. This is the best SEO possible. Waaaay better than any image filename which can contain from bad characters to cryptic numbers or word that have no relation to the real content of the article.
But here's another surprise!
If you're wondering why your images are not being indexed by Google, then I'm afraid Joomla is to blame. You see, by default, Joomla's robots.txt definitions block Google from indexing the /media folder which is used by K2 and every Joomla component known to man to store media files...
Simply delete that line in robots.txt and you should be fine.
K2 was designed to be SEO friendly since day 1.
And for that, K2 is priceless for many developers out there.