You have multiple options:
background-size: 100% 100%;
- image gets stretched (aspect ratio may be preserved, depending on browser)background-size: contain;
- image is stretched without cutting it while preserving aspect ratiobackground-size: cover;
- image is completely covering the element while preserving aspect ratio (image can be cut off)/edit: And now, there is even more: https://alligator.io/css/cropping-images-object-fit
Here are screenshots for some browsers to show their differences.
background-size: 100% 100%;
produces the least predictable result.