Is there an extensive source where one can look up artworks of Magic: The Gathering expansions?
Wizards releases some of the card arts in larger size in their wallpaper gallery, however I didn't see any Coldsnap art in there.

Other than that I would suggest seeing if the artist for the card you are looking for has their own website, since the artists do retain some right to their artwork. For example John Avon and Steve Arglye are both artists for Magic that have some of the artwork they've done at higher resolutions.