Smoke and fog FX can sometimes be tricky to get right. Here is a fog texture that i made that scrolls/tiles horizontally.
res at 1024 x 512.
I dunno how many fog effects i have made similar to this... probably 10,234. So hopefully this will save a couple people a little bit of time from reinventing the wheel from scratch.
if you like my assets, come to www.lfa.com for games, and some dev tools ^_^