For every 20 posts in tumblr, post a tweet with an image on twitter - work best with queue plus - so you don't flood your twitter!
If
Every N posts
This Trigger fires after you post a set number (N) of items on your public Tumblr blog. You may also select a specific type of post.
Then
Post a tweet with image
This Action will post a new tweet to your Twitter account with a linked pic.twitter.com image. NOTE: Please adhere to Twitter’s Rules and Terms of Service.
Discover more time saving integrations for Twitter and Tumblr (legacy)