Phase one:
Create a tumblr blog. Choose a niche and then think about a great name (search the web for this or visit the expired PR tumblr/blogs posts of this forum for having some inspiration.
Change your tumblr theme to minimal (or find, or type/customize something similar or more of your like). I you use minimal or stationery you can add an amazon widget on the right column.
Search on tumbr for blogs of your niche, and start to follow all that tumblr let you follow (tumblr has a daily limit of followings and at first this limit could be reduced if you have not content or followers).
Open a notepad/evernote or a sheet of paper and write a list of the urls of the tumblr blogs that have impacted you. They are going to be your source of info about what your niche followers want to see in a tumblr blog (study what kind of post get more interactions, which blogs has more followers/are more populars, is there someone featured by tumblr staff?, is there someone featured in the radar section?,...).
Make a sublist of the tumblrs that have daily photo updates.
Create a ifttt account and create a recipe for any new item at rss then create a photo post on tumblr. Using the blogs of the sublist. The best is posting them in queue and then review them before allow it to being posted. Add 5 relevant tags.
If your niche is SWF and you see, at the archive, that all posted is useful/interesting, automatize it a bit: making some recipes to directly posted and some to queue (configure the queue of your tumblr to 12 or 24 post a day).