MediaWiki:Common.css

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/* TOP-LEFT BANNER ONLY (minimal, safe) */
#mw-head,                /* legacy Vector */
.vector-header-container,/* Vector 2022 */
.mw-body-header {        /* other skins that expose a header */
  background-image: url("/wiki/Special:FilePath/BannerImage.png");
  background-repeat: no-repeat;
  background-position: left top;
  background-size: 420px 140px; /* adjust width/height to your image */
  min-height: 140px;            /* match the height above */
}

/* If your skin paints a base behind the header, make it transparent */
#mw-page-base, #mw-head-base { background: transparent !important; }