I am intializing CKEditor content by doing the following : CKEDITOR.instances.mytextarea.setData('<img alt="" src="myImage.jpg">');
The image wasn't diplayed by default and after a few hours of investigation I found that if I add 'Smiley' in my toolbar (in config.js) the image IS DISPLAYED!!!
I have no idea why 'Smiley' has anything to do with the diplay of the image but it works.
I'm using CKEditor Version 4.3.3 • 25 Feb 2014
I posted this because I lost a lot of time finding the issue and I think it might be useful to others.

It seems that you don't have
It seems that you don't have the image plugin enabled in your editor. Then the Advanced Content Filter strips it. Enabling smiley plugin helps because it also enables images, just like the image plugin.
Piotrek (Reinmar) Koszuliński
CKEditor JavaScript Developer
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Thanks for your answer
Thanks for your answer reinmar, but even with the plugin image I don't have images displayed in the editor!!
I tried with the Full CKEditor version and the following config.js :
CKEDITOR.editorConfig = function( config ) {
config.toolbar = [
{ name: 'basicstyles', groups: [ 'basicstyles', 'cleanup' ], items: [ 'Bold', 'Italic', 'Underline', 'Strike' ] },
{ name: 'clipboard', groups: [ 'clipboard', 'undo' ], items: [ 'Cut', 'Copy', 'Paste', '-', 'Undo', 'Redo' ] },
{ name: 'paragraph', groups: [ 'list', 'indent', 'blocks', 'align', 'bidi' ], items: [ 'NumberedList', 'BulletedList', '-', 'Outdent', 'Indent', '-', 'JustifyLeft', 'JustifyCenter', 'JustifyRight', 'JustifyBlock' ] },
'/',
{ name: 'styles', items: [ 'Styles', 'Format', 'Font', 'FontSize' ] },
{ name: 'colors', items: [ 'TextColor', 'BGColor' ] },
];
};
I'm using Chrome 33 and images are not displayed. But if I just add 'Smiley' they are.
I do not want the user to be able to add images, that's why the image plugin is not in config.js (but it does exist in CKEditor since I use the FULL package). I just want to have images in the initial HTML at startup.
If you have any idea so that I could get rid of the smiley plugin that would be very nice of you.
Having image plugin included
Having image plugin included in your build is not enough. What matters for the ACF is which buttons are enabled in the toolbar, or if feature does not have toolbar button, then it registers itself manually. So, as you don't have 'Image' button, the feature is not enabled in the ACF and therefore images are stripped.
You can extend ACF settings manually - to do that use the config.extraAllowedContent option. For more please refer to the the Advanced Content Filter guide.
Piotrek (Reinmar) Koszuliński
CKEditor JavaScript Developer
--
CKSource - http://cksource.com
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Follow CKEditor on: Twitter | Facebook | Google+
Your answer is exaclty what I
Your answer is exactly what I was looking for.
Thank you.