fullsize picture does not maintain aspect ratio

[hkjpotato]

The description in README is a bit misleading for FullSizePicture

"It will automatically fill the parent element maintaining the original image ratio. Please note that the parent element needs to have a defined height as you would expect for any background image as well.“

By default you are extends it to cover the whole slot, and the slot has a fix height, how can you maintain the aspect ratio?