It sounds to me as though you are making things a little too complicated for yourself. You don't need to name each individual photo or click the [Use selected photo] button on each photo in the synth.
All .jpg or .jpeg images which are dragged into the synther will be uploaded in the synth. The [Use selected photo] button is only used to choose which photo you would like to use for the synth's thumbnail image on the Photosynth website.
Adding tags, adding a description, and choosing a privacy setting are nice things to do, but you can always add or change them after the synth uploads to the site. They aren't needed to upload the synth.
Things which cannot be changed after the synth is uploaded are:
Changing which photo is used as the synth's thumbnail,
Changing the license on your photos in an existing synth