This is a really simple one this show an image placeholder like this if no image exists for this image field.
You will need to find the field number and add this in. Also you can change the url of the image and change the size.
CSS
/*tables*/
.field_* {
background-image: url(https://encrypted-tbn0.gstatic.com/images?q=tbn%3AANd9GcQ4HtE_O8tvs-TlF27vWMWHjxoCQ7HmFmZHBkZpKt1n4PFIN-aN&usqp=CAU)!important;
background-repeat: no-repeat;
background-size: contain;
}
/*lists and detail views*/
.field_* .kn-detail-body {
background-image: url(https://encrypted-tbn0.gstatic.com/images?q=tbn%3AANd9GcQ4HtE_O8tvs-TlF27vWMWHjxoCQ7HmFmZHBkZpKt1n4PFIN-aN&usqp=CAU)!important;
background-repeat: no-repeat;
background-size: contain;
}