If I understand correctly, you are able to browse to these files on the local machine, but not on the remote webserver. Is that correct? That would imply that the files are not getting uploaded correctly.
The log indicates that the files are getting uploaded, although perhaps not to where you expect them to be. For example, there is an entry for mobile/daywind.png that looks like this:
2025-06-23T00:00:44.171814+10:00 pc-case2-mint weewxd[387063]: DEBUG weeutil.ftpupload: 62 ./mobile/daywind.png 53ce0557ec7c98b057081345c912b8385b38a314be0ae777aa44ea96d55a5e31
2025-06-23T00:00:44.530225+10:00 pc-case2-mint weewxd[387063]: DEBUG weeutil.ftpupload: Uploaded file /var/www/html/weewx/mobile/daywind.png to mobile/daywind.png
I'm not sure what the long hexadecimal number (starts with 53ce...) is coming from. Is that something you added?
Is the regular Seasons report appearing on the webserver as expected?
Do you have shell access to the webserver? If so, poke around in there and see where things are residing.