There are many ways, but it also depends on what you plan (want) users to do. Since you mention that you don't want to be explicit about it, I assume you have some ideas about the reasons (which probably have to do with avoiding users leaving the journey because of a distracting element).
So, the most direct way would be the one that's been around since HTML existed: just add a border with the same link color. That's an affordance most users will recognize at first glance.
Another way would be to add a hint based on the content. For example, if the user goes to another page after clicking on the image, you can add the symbols commonly used for "new page", see the Unicode versions below
⧉ ❐ ⍈ ⎘
In the same way, if the user is just expanding the image, you can use any common "Expand" icons, such as the ones below from FontAwesome, which are easily recognizable as an expand affordance since it is present in many apps and websites, including Facebook and Youtube
Another option would be to use user discovery and learnability (not to be confused with discovery in the research phase). Basically, let the user discover the functionality by appealing to intentionality. In your case, this is very easy because most users who want to see if an image is clickable will actually click it. You don't have to tell them, but you can explore this behavior with any heatmap application or script.
If you want to support discovery with a more direct approach, you can use on-hover interactions (second click on mobile) and then add an overlay with a button, icon, or word that tells users that the image is clickable. This approach will work perfectly because the first time they hover over an image, they will see that interaction and immediately learn the affordances of the system.
Finally, you can use the least likely approach: a first-time onboarding process. This isn't very useful unless you can add other elements that require explanation, but I mention it because it's still an option