I hope you know CSS.
That plugin should do it however you may need this plugin as well.
in order to load a custom CSS into your project.
The HTMLElement isn't exactly user friendly and it'll take some work with CSS to get it right but it should be achievable. You should also be able to set the HTML to call a function in C3 when an image is clicked.
Inside your <img> tag you'd add something like this:
I haven't tested it specifically with images but it should work.
As for transparency, and I haven't tried this, there is an 'attribute' field under HTML for the HTMLElement plugin. I'd try
and see if that works. It SHOULD default to transparent.