Action Details
Requirements and how-to articles for Tumblr's action:

Requirements
You need this to get it done:

Create Call-To-Action
Include a call-to-action which you want your audience to do (maximum of 30 characters.)
Create Dashboard Banner
Can either be a photo (.jpeg/.jpg or .png, maximum resolution of 1080 x 432 pixels) or an animated GIF (maximum resolution of 540 x 216 pixels) (both options have a 3MB file size limit)
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How-To Articles
A list of useful links for you:

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Verizon Media Ad Specs - Tumblr Sponsored Day
Deliverables and specifications for a Sponsored Day promotion on Tumblr.
Official
Link missing here or found errors? Improve How-Tos