These designs are design speculation from an unknown artist of the three Konosuba characters based on their descriptions from the web novel before the anime adaption was a thing. The original source at https://www.pixiv.net/member_illust.php?mode=medium&illust_id=36284370 has since been deleted with no backup over at the Wayback Machine, so third-party source and artist request it is.

Below a version of the characters stacked vertically showing a bit more of their design in the width, taken from a website hosting Russian translations of the novel.

Erisorder.org

See also:
Reddit: The original Konosuba character designs.

And a very low resolution mockup cover page from a Spanish translation from Wattpad that used the same images, but a different one for Megumin.

Wattpad.com

This will be konosuba in 2013