Make sure you are integrated with Tumblr before submitting a photo post.
In addition to creating and submitting your Tumblr blog posts through Kapost, you can also create and submit Tumblr photo posts. This has a slightly unusual setup because you need to create a separate content type to do this - Tumblr blogs are created as blog posts, with an HTML body type, whereas photo posts have "photo" body types. If you have any questions, please contact your CSM at Kapost to walk you through the steps.
Once you integrate with Tumblr you must Add your Tumblr blog as a primary destination.
Create a custom field for your caption. It should be an HTML body type. Also, make sure that you put kapost_tumblr_photo_caption in the field name, regardless of what you put as the display name.
Don't forget to add the custom field to your Tumblr content type.
Now you can start making photo posts for your Tumblr. Create a piece of content, and select your Tumblr photo post types as the content type.
When you are on your content page, upload your photo and add a caption. Kapost will ask you to fill in a title - bear in mind that Tumblr photo posts aren't formatted to have titles, so this title will be lost in your actual post.
Hit publish and select your destination.