From their website.

Upload via FTP
Have lots of images? Upload image files only - no folders - to the Bigstock FTP server:
1) Install a FTP client (https://filezilla-project.org/)

2) Login
Hostname: ftp.bigstockphoto.com
Username: <your Bigstock username>
Password: <your Bigstock password>
You may use the FTP Log to check upload status of images.
3) Drag and drop

Important Information about Uploading
Do not use our FTP site to upload model releases.
You must upload your model releases using the Release Manager.
After your upload, go to Edit and Submit Images to add keywords, assign releases, write titles, descriptions and categories. When you're done, submit your images to Bigstock.
Photographs should only be uploaded in JPG format.
Illustrations may also be uploaded as PDF, PNG and EPS files.
The maximum size for image files is 25 MB.