Hi rubeus,
You are not doing anything wrong. That was a bug for any file added that passed it incorrectly through an xml repair routine (instead of an xhtml aware repair routine like gumbo) that was introduced in Sigil-0.8.900 series someplace that is fixed in Sigil-0.9.1.
The xml parser had no xhtml dtd to work with and so did not understand that img was a void tag (self-closing) and thus it introduced the error.
I have a release build of Sigil-0.9.1 on my machine (ready for a release after US Thanksgiving) and your plugin code runs quite well on my machine.
Here is the output of your plugin code running on the Sigil_Plugin_Framework_rev6.epub:
Code:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Allpics</title>
</head>
<body>
<div class="cimg">
<img alt="" src="../Images/Manage_Plugins.jpg" style="max-width: 880px" />
</div>
<div class="cimg">
<img alt="" src="../Images/Sigil_Plugins_Menu.jpg" style="max-width: 904px" />
</div>
<div class="cimg">
<img alt="" src="../Images/PluginRunner.jpg" style="max-width: 791px" />
</div>
</body>
</html>
If you try the Sigil-0.8.6 build or maybe some later builds, you will see that bug did not exist. If you use a Mac, I would be happy to privately post a version of the forthcoming Sigil-0.9.1 for you to continue development with. If you have Linux, and build from source (github.com master) you should also be good to go.
Sorry about that. I inadvertently introduced that bug when fixing another plugin related bug.
KevinH