Turn off new Tumblr beta post editor

Recently due to changes on Tumblrs posting you may encounter a small issue where photo posts are displaying as a text post would. Symptoms include container box having a scrollbar or blank space around the image.

You want to ensure it looks like this when creating a new post;

The fix is to ensure that:

To turn it off: Create a new TEXT post on https://www.tumblr.com/dashboard
At the top right look for the BETA sign and switch it off (grey).