Danbooru said:
Posting one's own art is severely looked down upon, and there are two reasons behind this:

Firstly, artists tend to be biased when it comes to judging the quality of their own work. What looks good to you may be regarded as mediocre to Danbooru moderators.
Secondly, allowing posting of own work (self-posting) will cause a sudden spike of sub-par uploads as users mistake us for another Pixiv or Deviantart, far beyond the scope and capacity of Danbooru.

More suitable sites to upload your own art to include Deviantart and Pixiv.