Posted on: 10/24/16 07:01PM
If it can't be found on IQDB and isn't on the first few pages of recent uploads, it isn't on Gelbooru.
You can manually check whether an exact image is on Gelbooru by taking the image's md5 checksum and searching for it. You can get the image's md5 using programs like md5summer. For example, the md5 sum of the fullsize version of that image is 4520540074fe2e7e1af4a8f1c2f3339b. You can search for that exact image on Gelbooru using the md5: metatag. So, for example, to search for the Pixiv fullsize version of that image, you would use:
md5:4520540074fe2e7e1af4a8f1c2f3339b
This returns nothing, so the fullsize image is not on the site. Repeating it with the sample version has the same result. If you're trying to upload the image and it's telling you that it already exists, this means that the image (or one with an identical md5 hash) has already been permanently deleted from the site.
If you remember seeing the post before on Gelbooru, it's possible that an alternate version was uploaded at some point. If you're unable to find that, there's a decent chance it's been deleted as well. It is, after all, by a non-Japanese artist.