At present, you will need to use a device with a touchscreen, most usually a tablet, if you want to zoom in on an image, perhaps to view an aspect of detail. On most laptops, however, this isn't possible. We are working with the developer on a solution for this, and hope it will be included in his next version of the Zenphoto software. In the longer term, we hope to introduce a greater number of zoom steps, using "plus" and "minus" buttons.
Also note that when you click on the title under a thumbnail image, you will be taken to an enlarged version of the image on a white background, with the Title and Description displayed underneath. This is actually still a compressed version, and you should click on the image itself if you wish to view the full-size image.