There was a problem in f-spot sending email photos, where the image was put into /tmp but was deleted after just 30 secs if you did not press the send button in your email client fast enough. I wonder if the same problem occurs with exporting to Flickr
It was necessary then to lengthen the time in a gconf key gconf/apps/f-spot/export/email/delete_timeout_seconds, though the key did not even exist by default, and had to be added manually by right clicking in the right hand pane of gconf-editor and giving it a decent length timeout value. I don't know if f-spot uses gconf now as dconf seems to have taken over from gconf for many apps, but it may be a good place to start looking.
Xubuntu 16.04 user, and loving it!