Photos as links to larger image

Using the Portfolio Press child theme, I have a blog post that includes an image I downloaded from the web. When I hover over this image, it becomes a link. Clicking on it displays the image in almost full screen mode. Good.

However, if I include a photo from my own camera, or an image I scanned, it displays in the post but is not clickable. Both files are jpg. Why?

Now, I want to use Twenty Fifteen child theme for a similar project. I’ve tried this with both pages and posts. What is the best way to display a large number of images (mostly scanned photos) so the reader can get a bigger picture rather than just the smaller image I use in the text? I thought of using a separate page (or post) just as a “library” for the reader to look at the scanned photos. This is for displaying family history documents. I’d like the reader to be able to click and see a larger image after reading the page (or post), without them having to download, save to their pc, then zoom in on their picture viewing software.

Thank you kindly for any thoughts.