Welcome to the "advances" of the macOS.
This is a known problem and one of the reasons I delayed as long as possible "upgrading" from 10.11.6 on the mini that was used exclusively for WC. Unfortunately, that mini went "blind" and the user interface was no longer accessible. Naturally, it was replaced with a computer that came with 10.13 installed.
I suspected I would face this problem and I have. My immediate "fix" was to simply change the nav menu for all pages referring to the page that displayed those fine and very easily modified images from my web site. It's a shame to have all the work done by the Developer to create the very nice UI for building/editing those gauges go for naught.
They reside at ~/Library/Application Support/WeatherCatCustomWeb/ Unfortunately, even if you look at them there, you won't see anything. They are just a small .png file with no visible image. It's probably simply a transparent square (250 pixels wide and tall) that serves as a background for the actual gauge.
They update according to what page you have the WC tags located and how often you have WC updating that page. Therefore, due to the bug that Apple has been told about for longer than 10.13 has been around, there really is nothing you can manually upload, at least efficiently.
There seem to be only a few of us who have to deal with this problem. The vast majority here use the Steel Gauges templates which uses javascript to create the gauges. Be aware that some of your viewers may have disabled javascript for security reasons, so they won't see the gauges, even if you do.
Some links to this discussion:
Wish I could be more helpful. I may investigate the drawing tools now available in CSS3 which can now replace lots of javascript actions. It also now has variables which could be helpful in building images.