Copyright wise, yes, strictly you'd need to email to ask permission. Practically speaking, however, you won't get in trouble for using it as it's unlikely, if not impossible, that this will ever get any attention from the original "artist", yet alone negative attention, so you will be fine. I've never heard of any case where a minor copyright infraction used on a non-profit work has lead to any trouble. Ideally, it's often best to look through a dedicated texture archive that releases it's content under Creative Commons or some similar lisence - you're also more likely to get better and higher quality images from those sorts of sites.