Same problem using Opera Developer. I can't upload a new picture to a board I am sharing although I can change existing items (not pictures.) If I use the approach suggested here, unshare, change, reshare, that works fine as far as the added image goes.
Unfortunately, while you can make changes to an existing board and even add new links, you can't add new pictures in either case.
If you unshare the board, add your new image, and reshare, it creates a new board with a new URL that you have to send to everyone you've shared it with. The previous one just goes away along with any previous activity.
In short, I think Opera just isn't done with this and we'll have to wait until they get around to finishing it.