MediaWiki:Common.css

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/* Top-left banner (final) */
#mw-head,                /* Vector (legacy) */
.vector-header-container,/* Vector 2022 */
#p-logo,                 /* Monobook/Modern */
.ts-header {             /* Timeless */
  background-image: url("/Special:FilePath/BannerImage.png");
  background-repeat: no-repeat;
  background-position: left top;
    background-size: contain;   /* or use "cover" if you want it to fill the box */
  min-height: 200px;          /* make space for the full banner */

}

/* Remove base header strips some skins add */
#mw-page-base, #mw-head-base { background: transparent !important; }