Firstly is it saved as a jpg?
This may sound funny but you may also need to clear your cookies from your computer (in your settings you need to select browsing data > cookies > clear). Sometimes doing this will help.
Did you try a couple of times? I have had to do this process sometimes. Meaning I would select another photo, see if that loaded then go back to the one I wanted to post and select that again. Crazy I know but it works.
Let me know how it goes. If no luck email me your photo apistikakis@bcna.org.au and I will add it for you.
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