It's been a while since I worked Tumblr but yes I know that NSFW thing caused some problems and made it harder for a while than it was a year ago. Mainstream could work out on it but I don't think something like Oreos would work too well though that is just an example. You'll want to try to make it go viral so something funny or shocking in some way would have a far better chance. That is what I would shoot for.
The thing about mainstream is with some niches you still have a good chance to make some money even if you can only get 50 people clicking through a day. In adult 50 people clicking through every day might buy you a grape after a couple weeks.
Copyright is the same a you would normally expect. If you didn't create it and you haven't bought the rights then technically you can't post it unless you have permission. But I haven't heard too much about Tumblr taking a lot of stuff down for copyrights. Copyright infringement is rampant on Tumblr.