It's related to the configuration of Steemit's ImageHoster (https://github.com/steemit/imagehoster). You can't upload images as long as your account has a reputation < 10.

Defined in imagehoster/config/default.toml

[upload_limits]
duration = 604800000 # in ms (=1 week)
max = 300 # max requests within duration
reputation = 10 # minimum reputation needed for upload