Cover image is blurry on some (not all) smartphones

On this homepage I use a cover block with an image.
https://www.dunjajocic.com/
On desktop the photo with the dancers has good quality. On some smartphones its blurry.
I guess it has to do with the media queries but can’t find where I have to adjust this.
In the browser inspector I can see there is a 62KB photo shown on a screen that has a width of 400pix. At that width the quality of the photo has to be better. But how can I manage that?

Thanks in advance for your help,

Moon