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I believe the current setting is 1200x1200 pixels or 5MB in file size. Do you know the dimensions or file size you are attempting to upload?
I don't know the exact size but it's just a regular picture taken with my iPhone. I have tried several different ones (planned on doing a reveal!) but they all fail because they are too large. I think the App allowed us to change the resolution as we were uploading the pictures but I can't seem to find a similar option on the mobile site. That's kind of a big deal if we can't upload iPhone pics easily!