ePub Hidden Cover Image

[LostOnTheLine]

So I just edited a fan-translation ePub to downsize the images (Extracted with 7zip, OEBPS --> Images, resized each to 1024 hight & rotated wide pics) then I re-ePubed it (7zip --> Add to archive, zip/deflate/fasted compression, changed extension to .epub) which I've done many times before. But when I added it to Calibre it showed a different version of the cover image with a size bigger than any of the images in the file. I tried removing the cover & adding from the epub, but it shows the same image. I cannot find that image in the folder structure at all. I have hidden files shown, even system files & protected operating system files are shown, where is this image coming from? none of the other files I can see are even larger than 15KB

I know I can just have calibre resize or replace the image, but I really want to know where the image came from. Any ideas?