We were applying a white background to transparent images
and converting them to jpegs. This was to make text be legible
behind a black preview background.
However, this has led to a poor user experience, as users rarely
download the full image but always click on previews. Therefore,
we need the previews to remain as pngs.
To fix this, we just re-encode them as pngs instead of jpgs.
```release-note
NONE
```
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