Requested feature overview description.
This would allow a user to upload or link a URL for looking up an image the same way google does image lookups. Since this website blocks google spider via robots.txt, google can't spider the site looking for images.

Here is an example: http://tinyurl.com/furryimagelookupexamplee621
I was trying to find the original of this image, and luckily I knew the artist, so I found it even though image lookup on google failed.
https://www.furaffinity.net/view/8569579/ <- And thus I was able to go favorite it.

Why would it be useful?
Tracking down who made an image when you only have a partial or censored version, so that you can fav the original and possibly even commission the artist.

What part(s) of the site page(s) are affected?
"Posts" is probably really the only one really affected, and the home page with an image lookup icon.

there is already list of reverse image search buttons on every post page. also we cant do here anything that would help with reverse image searching things from furaffinity.

I don't see a reverse image search button, unless you mean the ones on the submitted art. I want an image search to find an image here on the site, as if I put it into google, but instead I upload it to the search feature here for it to see if the image exists or similar images exist on the site already. Searching "green dragon" may yield thousands of results when you're looking for one specific thing in particular, like trying to find if the image you found somewhere else, is already here.

Vas said:
I don't see a reverse image search button, unless you mean the ones on the submitted art. I want an image search to find an image here on the site, as if I put it into google, but instead I upload it to the search feature here for it to see if the image exists or similar images exist on the site already. Searching "green dragon" may yield thousands of results when you're looking for one specific thing in particular, like trying to find if the image you found somewhere else, is already here.

Google robots do go trough this site, so using googles reverse image search should also give results from here. If you want to only search this site specifically, use site:e621.net on search bar.

Then there also is site dedicated to only reverse search here: http://iqdb.harry.lu/

Also side note: apparently google reverse image search was able to detect "penis" and "breasts" in few images I tried just now, when earlier the guesses have mostly been "cartoon" or "anime". They are evolving :|