I suspect this to be a refactoring relict as the bootstrap for this theme declares info.magnolia.module.extendedtemplatingkit.imaging.ETKImagingSupport as the class for the imaging node. I expect info.magnolia.module.templatingkit.imaging.STKImagingSupport to be the right choice.

As a workaround someone can create a pseudo-implementation like this:

package info.magnolia.module.extendedtemplatingkit.imaging;

import info.magnolia.module.templatingkit.imaging.STKImagingSupport;

public class ETKImagingSupport extends STKImagingSupport {
}