This is what I did. I have a sequence of camera photos with gps info included using geotagger (gpx files obtained from smartphone). After logging onto mapillary page, just upload the whole collection of photos. Mapillary will ask for some details prior uploading such as whether to load as a sequence, camera heading angle etc. Then upload. Takes quite a while if you have 2-300 pics, depending on file size, bandwidth and their server.