You should be able to right click (or tap-and-hold) on the image and choose something like “save image as” in order to download the file, or “copy image URL” to copy the URL to your clipboard. You can then paste that URL into a Lemmy comment with the image embed syntax like so:
After testing a bunch of things, I have to conclude that is a Voyager issue. Uploading using Thunder or just the normal Lemmy UI in a web browser works just fine no matter what instance I use. But using Voyager, I have the same JPEG issue as you do.
New fav gif
And now I have to figure out how to deal with these web gifs
You should be able to right click (or tap-and-hold) on the image and choose something like “save image as” in order to download the file, or “copy image URL” to copy the URL to your clipboard. You can then paste that URL into a Lemmy comment with the image embed syntax like so:

Uploading with sync fails and Voyager sends a JPEG…
After testing a bunch of things, I have to conclude that is a Voyager issue. Uploading using Thunder or just the normal Lemmy UI in a web browser works just fine no matter what instance I use. But using Voyager, I have the same JPEG issue as you do.