When i insert a picture, the field "Popup Type" under register Popups will show "JCE MediaBox Popup" - but in the field URL will not set the adress for the popup-picture.
When creating a popup in the Image Manager Extended there are 2 image URLs, one for the thumbnail or target image that the user clicks on, and the second for the image that is to be displayed in the popup.
Once you have selected the first image, in the Image tab, click on the Popup tab and select JCE MediaBox Popup from the Popups list. This will automatically fill the URL field with the same value as in the Image tab.
Alternatively, click in the URL field in the Popup tab and select the same or another image to fill the field with the image url value.
Please see - https://www.joomlacontenteditor.net/support/documentation/image-manager-extended/creating-popup-images
Ryan Demmer
Lead Developer / CEO / CTO
Just because you're not paranoid doesn't mean everybody isn't out to get you.