When it comes to the web, we almost have to tell users what to do.
Images have no indication that they are links.
Unlike page links do . So we need to tell them to click. Add a small piece of text that tells them to click, usually on the bottom right corner. An itchy trick you can do is place a small dot next to it, sort of like a target for the eye or button. Some pictures on the web need a definition of what it's for.