I always like Ensiferum's covers:
Heroïsm, battles, paganism, folk stories - it all comes storming into your mind the very second you see this cover. There's no better pic to go with the album's title "Victory Songs".
Korpiklaani - Karkelo
Warm, cosy, nostalgic setting. Storytellings on a cold winter's night, while sitting near the campfire with a good glass of Vodka (which happens to be a single from this album)
Eluveitie - Spirit
A pagan, nostalgic theme. About ancient, lost times. Celtic.
This cover too fits the album theme perfectly. Eluveitie's music is about the Gauls, with their lyrics often in Gaulish too.
Amon Amarth - With Oden on our Side
Simple design, and that's where its strength lays. There's nothing to draw attention away from the band/title and the two pagan symbols (the Valknut and Odin on Sleipnir)
Sirenia - An Elixir for Existence
Beautiful color use. Here too a bit of cosyness. Alcohol
Do I need to say more?