Class deleted from colorbox image

Everytime I save my content with Full ckeditor loaded the class="image-large" is lost. I tried setting up config.extraAllowedContent += 'code[language,class];img[alt,src,width,height,class];'; without success. I also tried config.extraAllowedContent += 'img[*];'; both via config.js and also Extra allowed content 'img[*];' via ckeditor config form. All fails.

Just a side note... not sure if related - I have image2 plugin installed.

Before save:

<a href="/files/foo.png" class="colorbox colorbox-insert-image" rel="gallery-all"><img src="/files/styles/foo.png?itok=bd3cvkWy" width="425" height="480" alt="" class="image-large" /></a>

After save:

<a href="/files/foo.png" class="colorbox colorbox-insert-image" rel="gallery-all"><img src="/files/styles/foo.png?itok=bd3cvkWy" width="425" height="480" alt="" /></a>

What is the root cause of this and how can I stop this?