To use Glass-to-Flickr, you'll necessarily need to authorize with both Google and Flickr. Glass-to-Flickr will only receive photos from Glass when you explicitly share them with the Glass-to-Flickr "sharing contact" and will only post or update photos on Flickr in direct response to such an action. You can revoke these credentials through your Google or Flickr settings.
I keep your Google and Flickr usernames in the database along with the OAuth credentials for each service. Logs are also kept of each visit and each time a photo is shared; I consult those logs regularly in order to debug and improve the service and may also create aggregate statistics of usage. The actual content of photos is transferred through this service, but never retained more than transiently, unless debugging a specific problem.
You should be aware that this is experimental software and a side-project for a single person who does not consider himself an expert in network security. Still, I take privacy seriously and will not intentionally use your data contrary to this explanation. Furthermore, I'll try proactively to prevent any accidental disclosures or external abuse of this system.
P.S. Please contact me with any questions.