Could you please double-check whether the attachment directory exists? Also, double check whether you have /home/nemo or /home/defaultuser. The error you get on receiving is really associated with creating the attachment file, and the parent directory not existing. Maybe really check character per character, or re-enter the path yourself to really make sure.
Wrt. transmitting a photo, I feel like you’re doing this without having restarted Whisperfish when something else has already failed (Mailbox has closed), so maybe let’s first focus on your first issue 