Steps to reproduce:

1. Download https://phabricator.wikimedia.org/F20970 , a PNG image file with transparent background
2. Upload via http://phorge.localhost/file/upload/
3. See that its page http://phorge.localhost/F220 shows a checkered background for the image; hovering shows full contrast.
4. Create a task with the description {F220}.

Expected outcome:
Image is rendered as on page http://phorge.localhost/F220

Actual outcome:
Image is purely white; hovering changes nothing:

Other comments:
Defining the same behavior for .phabricator-remarkup-embed-image img as already for .document-engine-image img in webroot/rsrc/css/phui/phui-property-list-view.css leads to the following results:

Downstream ticket:
https://phabricator.wikimedia.org/T78644