Which subscription will be the right one for: only regularly uploading (every 6 minutes) a small image - about 30 kB (no video)

Your question seems to be about CameraFTP.com. CameraFTP is a DriveHQ subsidiary, but it is a different service from DriveHQ.com. Please visit www.cameraftp.com for more info. 

Based on your info, you can order an image recording plan. You can fully customize a plan based on your requirements and budget. Please see the screenshot below:

 Thank you. This solved my question.