Pinterest: Create A Board and Add a Pin
If you see an empty board with no pins after clicking the 'Pin it' button, don't worry.
Go back to your profile.
Find your board and click anywhere on it.
Click the large empty area that says 'Add a Pin'.
Click the big arrow.
You will then see the 'Choose image' button again. Click the 'Choose image' button and you will see the Windows file selection dialog again. Add an image as you did previously.
That's it. You're done!
I tried to cover an alternate path to uploading a pin in this tutorial. If the information is confusing, please let me know what is unclear and I'll modify this tutorial or break it up into 2 separate tutorials.
I hope this helps someone.