Requested feature overview description.
The site's internal IQDB image search program now extends to the upload page. More specifically, it can be used to reverse search images on the upload page's cache, before a user uploads it to the site proper.
OPTIONAL: A notification of some sort if the uploaded image resembles a previously uploaded piece too heavily?

Why would it be useful?
If a user uploading an image doesn't have a source handy, or simply never noticed the harry.lu feature before, they could use this function to see if their image has been previously uploaded, potentially reducing the number of duplicate files that have to be flagged and deleted.

What part(s) of the site page(s) are affected?
The upload page and possibly the harry.lu service. Come to think of it, does the FAQ have a description of e621's various reverse image searchers?

harry.lu is actually an independently operated site provided by yesthebest for the benefit of the site. We don't have any direct control over it aside from providing the links on the posts page.

That being said, as much as we appreciate them providing the site for us to use, running our own version of IQDB server-side might not be out of the question.