otaku_emmy said:
We have a rule that anything that isn't approved for more than three days gets deleted. Usually we don't undelete them.
There is a forum #45909 (Shaun of the Deleted, Deletion Appeals) which as the name states is to appeal deletions.

Usually images that have aged the 3 days to deletion meet the quantitative guidelines.

The images may be borderline in some way such as the level of jpeg artifacts or poor lighting, or not a good as a wallpaper, or the moderators felt the image lacked something in esthetics and did not approve it - leading to the automatic deletion.

If a compelling reason is provided, many times such images may be undeleted.