We have consitently had issues with new users to discourse not being able to post more than 1 image. Given that we are a visualization library, it is common people will want to post before/after images which require more than 1 image (or them to merge their images into 1 before uploading which, just no).
I have unilaterally uped the limit to 4 media for new users. That is hopefully enough that a majority of new posters will not get caught, but will still bounce egregious spam attempts.
We can re-address this if it becomes a problem the otherway.