You can't, it's how K2 was worked since day 1. The filename is not really an issue when you have other complimentary data so that search engines can relate a search query with your image file. And by default, K2 will use the title of the item if you don't provide a separate caption for the item image.

If your images are not crawled by search engines, check if the /media folder is blocked in your robots.txt file (older Joomla versions blocked that folder by default unfortunately).

For the alt and/or title attributes of the <img> tags. This info is sufficient for search engines to classify your images, even if the filename is not relevant to the actual image (a hash).

Keep in mind that K2 v2.10 (which is coming in the next few days) also features structured data definitions which further assist in content indexing from search engines.

You're welcome.