The main difference between a filter and a backdrop filter is that the filter will affect the element itself, and the backdrop filter will affect the elements beneath the element within the bounding box of the element.
A sample difference with Blur() :
Filter:blur(2px)
backdrop-filter:blur(40px)
So, if you want a blur effect on an image, you have to give the CSS style to that image. Then you have to overlay contents separately.