Also, for something like that you should use <img> instead so it's less of an XSS risk.
Also, for something like that you should use <img> instead so it's less of an XSS risk.
Yes. But depending on how gmail works it could me made reliable and secure. For instance if you can share images with gmail users you could generate a unique image for that user, do the same thing with an <img> tag, access the image data with JavaScript, send it back to the server and compare if the contents are the one you shared.
I do not have a gmail account so I don't know if this is possible, it it seems like it would be possible for Google+ from briefly looking at it.
And if it was an image generated by Facebook, then Facebook must have access to the account beforehand, and there's no benefit to using this system over OAuth.
// EDIT: ignore what was here, you're right.