Adding Background Image?

When I try to add a background image using a downloaded image moved to the collection.media folder, and put

background: url(50a78b21b11300c8e71ec378ebdbaabf.jpg);

Nothing happens. However, if I try replay the image with any other image in the collection.media folder, it works. Putting the direct link to the image

background: url(https://i.pinimg.com/originals/50/a7/8b/50a78b21b11300c8e71ec378ebdbaabf.jpg);

Works too. But putting the actual file still doesn't work, even after syncing, and pasting the image into a random card won't get it to show up in the collection.media folder either. Please help.