Default image if the collection's cell is empty. Is it possible?

You can surely solve this by using some CODE.

  1. Quering your DB-Values
    https://www.wix.com/velo/reference/wix-data/query

  2. creating a → if-else-query

Pseudo-CODE…something like this…

if(value of db-field is empty) {show my predifined PIC}
else {show the current DB-Field-PIC}