The good thing about teenagers is they all want quick and easy money. If you search the niche using different related tags and you will be able to find some blogs that of a medium size. By medium I mean, the blog is getting a lot of likes and reblogs but they aren't the kings and queens with tens of thousands of followers because the people with huge accounts won't sell out. For example, Discolored won't "betray her fans for any amount of money".
So once you have a list of blogs, contact them! Send them an ask from the blog that you created at the beginning and ask them to check it out but ALSO say that you have an offer to rent their caption on each of their posts.
DEFINITELY ask them to look at your blog in the message as it looks a lot less spammy and really improved my conversions.
If they respond saying that they're interested then you can offer to buy the entire account because of course that's cheaper for you in the long run.
However if you buy it, you have to manage the blog and Hipster accounts aren't really able to be ran by scraping tagged photos. These blogs have been built up from Q&A sessions and different activities with a real account owner that the followers have become to know so although more expensive it is worthwhile keeping the owner posting.
I've bought accounts with 10,000 followers for about $50 when the owner was really desperate for the cash but for ease, I generally rent the blogs.
I pay $20/month on average but will always start at $10 because after all it's free money to them. As I rent quite a few blogs one of my terms is that the owner must contact me at the end of each month to get their payment and if they forget then that's their loss. Many people will forget after a few months and keep you caption there so it's free traffic for you!
Then if they remove your link you can contact the owner again and renegotiate or just remind them.
So that's how I make an easy and autopilot income without having to rank any sites or build a fan base.
Of course the Hipster niche is just one area you can try. Tumblr is a huge place so you can't saturate this and there are plenty of other niches to try out. All that could happen is that the price for rental goes up which is life! Play the game to win the game!