My feeling was right. Checking the web for this issue I found
The first link contains a script on how to copy the hidden src attribute which is saved by CKEDitor into the src attribute that your plugin is modifiying,
It looks like this and should be part of your plugin, I think.
CKEDITOR.on('instanceReady', function (ev) {
var editor = ev.editor,
dataProcessor = editor.dataProcessor,
htmlFilter = dataProcessor && dataProcessor.htmlFilter;
htmlFilter.addRules( {
elements : {
$ : function( element ) {
if ( element.name == 'img' ) {
var imgsrc = element.attributes.src;
element.attributes.src = imgsrc;
element.attributes['data-cke-saved-src'] = imgsrc;
}
}
}
});
});