K2 also has big issues with the cache system. It generates a lot of cache files and you will quickly realize you need extra hosting space. For every image you upload, K2 generates an additional 4 or 5 cache images that take lots of space. Again, this issue is more than 3 years old and no fix or update.
I would strongly recommend against using K2 until these basic issues are fixed.
K2 creating multiple sizes of an image is not a bug, it's a feature. Furthermore, K2's SEO is far better than more content components' is. Not only can you NOT have duplicate URLs by simply creating a hidden menu item pointing to all of K2 categories (this fixes duplicate URLs which would be normal to have depending on where you're browsing the site), we have also included an Advanced SEF section which can alter the URLs to your liking and even remove the item id creating URLs that only commercial SEF components can - and without the DB overload they bring.
K2, like all good things, needs some time and devotion to learn. If you can't see the benefits of it, then you might as well make a static website. I hear Jekyll is all the rage nowadays (good luck with that).