There is also a footnote:
This suggests to me that uploading a new PNG file for an existing entry is supported.‡ If you are not uploading a new PNG file or CFDG file for this entry update then do not click on the associated file buttons. The previously uploaded PNG or CFDG files will be retained if you do not upload a new one.
I even thought I had done it once years ago, but repeated attempts from Opera and Firefox, caches cleared, have had no effect on the displayed image. Except that it always forces a fresh download of the old image under a new name (actually the old name, with incremented index in the query string).
I can think of good reasons why that functionality might be disabled, but I don't know whether it is, or if there is a fault somewhere.
Deleting entries or posting multiple versions also have their downsides, so I'm hoping it doesn't come to that.