I dont think there is some restriction about that on Facebook .Not sure,you might have to check out
http://developers.facebook.com/policy/
Also Best way would be to store images in Users album on Facebook using extended permissions ,instead of your server so it seems legitimate to both users and Facebook.
Though don't know what type of facebook application you are creating where you need to store images on server but it will not be a problem till you follow their privacy policy and terms.
And may be this app is something you might want to check ,its in beta and may be on same idea , and from that you can be sure
http://usepipe.com