Do you have Facebook set up to sync your photos?
While that's one way to do it, that's not the only way and it won't allow you to be selective of what you want posted.
Assuming you have the Facebook application installed and no custom Gallery here's the procedure: To post manually go into gallery, long press a photo to begin the selection process, then start checking each additional photo you want to post. When all the photos you want to post are selected, click the Share icon (3 dots connected with lines) and share to facebook. Review the settings presented to you in the facebook app that comes up.