Version 7.3.0.0 available.
I've tried this in Firefox 85.0 / Win10: Insert - Image/video/file - press the grey Upload button under the empty File/URL box. Then an Explorer window comes up where you can select the image you wish to upload - press Open. The upload starts immediately.
So direct upload of a local file appears still to be possible. Sorry for my too shortsighted answer in first instance.
...now that we sorted the above, I'm glad I'm not the only one seeing this problem:
I've updated to 7.3.0.0 some hours ago.
Now sync with any of my devices fails.
When I click on device image on home screen an error message pops up saying something about communication error with servers.
I tried sync with mobile connect app which works, so I assume it's not a problem with Garmin servers.
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EDIT
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Well, I deinstalled and did a clean install. Working again.