You may have seen that some images on a glog enlarge when you click on them. There is no zoom button or function, whether the image enlarges is dependent on the size of the original image file and how you change it after adding to the glog, like this:

If you add a large image to a glog and then reduce the size and save the glog, after saving, the image will zoom (enlarge) to the original size when you click on it.

If you add an image to a glog and do not reduce the size, or even enlarge it, it will not zoom when you click on it after saving the glog.