WNDR4500v3 Firmware version v1.0.0.60_Beta

> I've followed the instructions to the letter [...]

   Sure you did.  _Which_ "the instructions"?  What, exactly, did you
do?

> [...] when I try to upload it [...]

   "it"?  The zip archive itself (wrong), or the firmware image file
(".img") which you extracted from the zip archive (right), or what?

> Afterwards I did open the zip file and indeed found that it contained
> an image file [...]

   "Afterwards" is too late.  "open" how, exactly?

> [...] Where is the file? [...]

   It's in the zip archive.  You need to extract it from the zip
archive.  If you're running some version of Windows, then right-click on
the zip archive file (in File Explorer), and select "Extract all..." to
extract the files in the zip archive.

      https://kb.netgear.com/23960

> [...] and a link to the site [...]

   That's not a link; it's the whole document.