Updating this part, by popular request.
The main characters are:
Protagonist 1: Norse viking, young man/teenager, blonde/fair hair, green/blue eyes, sparse or no beard, leather shirt, wields a sax (a long knife)
Protagonist 2: English/Saxon young man/teenager, dark hair, brown eyes, no armor.
Antagonist: Viking warrior, huge man, blonde hair, trimmed beard, vain. Reinforced leather armor. Wields a quite large, one handed sword and a shield.
There is an important scene where the two vikings (the protagonist 1 and the antagonist), duels on a Northumbrian beach following a raid, with the other vikings surrounding them. We've envisioned that scene on the cover, but other/creative designs that "catch the essence" of the book or the era will also be considered.