I was doing the screenshot one after another, and doing the blog at the same time. Everything went smoothly. Then I uploaded all the screenshots at one go. Following by filling my description for each photo, and completed the whole review post in less than half an hour. Very convenient and easy to use. More like a app version of full markdown web version editor.

I don't remember if I saved my active key or posting key in my steemlogin, my best guess is you'll need the active key to authorize the app. But you definitely do not need a master key for sure.