Change what you want to search for below if 'cityscapes' aren't your thing :) Post will also include proper attribution and a link back the 500px photo.
If
New photo from search
This Trigger fires every time a new photo is uploaded to 500px that matches a search query you specify. NOTE: limited to 30 items per check.
Then
Create a photo post
This Action will create a photo post on your Tumblr blog.