We put a real png image as a header

cara/apps/expert/cara.ipynb

@@ -4,11 +4,8 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "<div style=\"text-align: center;\" align=\"center\">\n",
-    "<img src=\"../files/static/images/cara_logo_text_cern.png\" align=\"center\" style=\"text-align: center; height: 12em;\" /></div>\n",
-    "<p style=\"color: blue; font-weight: bold; font-size: x-large; text-align: center;\" align=\"center\">\n",
-    "Airborne Transmission of SARS-CoV-2\n",
-    "</p>\n",
+    "<div style=\"text-align: center;\" align=\"center\" >\n",
+    "<img src=\"files/header_image.png\" style=\"height:8em\"></img></div>\n",
     "<p style=\"text-align: center;\">\n",
     "Please see the <a href=\"https://cara.web.cern.ch/\">CARA homepage</a> for details on the methodology, assumptions and limitations of CARA.</p>"
    ]