Add to extra.less
.msgBgByName (@msgBgAuthorName; @msgBgUrl; @msgBgSize; @msgBgPosition: false) {
.message[data-author="@{msgBgAuthorName}"] .message-inner {
background-image: linear-gradient(to top, @xf-contentBg, fade(@xf-contentBg, 60%)), url(@msgBgUrl);
background-position...