I get asked pretty often where to find good original content on Tumblr. Here is a short list of some places I go.
Always check of the photography spotlight at http://www.tumblr.com/spotlight/photography. There are always great photogs there. Another great place to find original stuff is at http://photographersdirectory.tumblr.com/.
Some of my personal favorite people are:
http://chrisvillacillo.tumblr.com/ He’s got great landscapes, black and white and some portrait work.
http://blog.noahkalina.com/ He always has great creative stuff.
http://everyday.joelzimmerphotography.com/ Joel always has great on the street work and man, this guy produces. He’s got new stuff every day.
http://www.ronniebruce.com/ Ronnie has a bunch of great stuff both professional and creative.
There are also a bunch of great curators and curated sites as well.
http://karishma.me/ has awesome landscape
http://black-and-white.tumblr.com/ is great for black and white pics
http://www.minusmanhattan.com/ has a great selection of artistic, tech and design stuff. He also shares some of his own stuff from time to time.
http://life.tumblr.com/ is a great place to find historical fun stuff.
http://tumblr.photojojo.com/ has a great selection of pictures and all camera related stuff
There are a ton of other great people out there. Sometimes they are hard to find but stick with it. They are there. Good luck.