There is a bug in codemirror field that the field does not save unless if you open the plugin editing page, it has been reported before, IPS need to fix it, and this is not a bug with my plugin.
anyways, open the plugin editing page in a new tab, simply right click on the pen icon and open in a new tab and then edit the custom html field and save
You can use the custom EDITOR field to add the image too
<img src="IMAGE LINK">