bbPress.org

#bbpress-forums .bbp-author-avatar .avatar-14 {
  width: 24px;
  height: 24px;
  padding: 0;
  border: none;
}

if that does not work try this

#bbpress-forums .bbp-author-avatar .avatar-14 {
  width: 24px !important;
  height: 24px !important;
  padding: 0 !important;
  border: none !important;
}