This has been implemented network-wide. When a user has <= 15 rep, they will see a message in the image uploader that says:
Images are useful in a post, but make sure the post is still clear without them. If you post images of code or error messages, copy and paste or type the actual code or message into the post directly.
If you have less than the new user privilege (< 10 on SO) and you attempt to post an image, we will now include a link to the image and a little message to the user:
Previously these users would get an error message when they attempted to post.
Note: the link conversion this will happen only on sites where image embedding is blocked for new users, i.e. on SO
Basically, we aren't preventing users from including screenshots; we are just trying to discourage low rep users from including screenshots by themselves.