The post formats are all at the top of the dashboard, when you click those big, colorful graphics. I actually had to fumble my way through learning how Tumblr works. I haven't found a primer, and the complication comes in browser add-ons like Missing-e and Tumblr Saviour which increase site functionality for the users who install them. I wonder if such a thing exists, though? I know I would benefit from it. There are still some things that I am sure I don't understand or know about. :-/