Hi mate, Steemit can be a bit of a mystery sometimes! I've figured out how to able full screen view only a month or two ago myself. The trick is very simple, you have to use markdown code.
Write the following lines and replace the "your image address" and "your bottom text" by the actual image address and text.
I hope this answer your question :)
Take care.
Great job! Thanks so much.
Thanks so much.
You are most welcome :)