I downloaded the latest shadowbox and used your code above and it worked fine.
Are you sure the paths are correct to both shadowbox and the image.
Have you checked the console logs?
In terms of setting it up so it opens on load they have an example on their website...
EDIT: Player needs to be set to "img" and the content only needs to be the path to the image.
<script type="text/javascript">
Shadowbox.init({
// let's skip the automatic setup because we don't have any
// properly configured link elements on the page
skipSetup: true
});
window.onload = function() {
// open a welcome message as soon as the window loads
Shadowbox.open({
content: 'https://www.google.co.nz/images/srpr/logo11w.png',
player: "img",
title: "Welcome",
height: 350,
width: 350
});
};
</script>