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The plugin iManager has such function.
First you have to insert the image with smaller dimensions (Properties).
Then select the inserted image, open iManager and select the Popup button from the left. Now you have to select the image again and after clicking Insert the thumnail is linkable.
If you now click on the thumbnail a window pops up with a larger scale of the thumbnail. You can click the image to close the window.
Please note that if you remove the image from the editor field, the code for the popup still exists. This have to be removed manually in html-view.