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

Post a photo on Tumblr.

You need this to get it done:

Static Image
File format: .jpeg/.jpg or .png
Maximum file size: 20MB
Maximum width: 1280 pixels (recommended no greater than 500 pixels)
Maximum height: 1920 pixels (recommended no greater than 750 pixels)
Colour model: RGB
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How-To Articles
A list of useful links for you:

Tumblr Help Center - Photo Posts
When you are logged in to Tumblr, navigate to the homepage (a.k.a. the "Dashboard") and at the top of the page next to your blog's avatar you should see a list of different types of media. Click "photo" and then choose whether you wish to upload a photo or use which has already been uploaded elsewhere.
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