Explanation:

- Option 1: "by" - This preposition implies an agent or means, like "by someone" or "by using something." It doesn't fit the context of pictures circulating.

- Option 2: "of" - This preposition typically denotes belonging or relating to something, as in "the picture of a cat." It doesn’t convey the correct relationship between circulating and the platform.

- Option 3: "to" - This suggests a direction or movement toward something, which doesn’t accurately describe the ongoing spread or presence of pictures on a platform.

- Option 4: "on" - This preposition correctly indicates a medium or platform where content is located, like "on social media," making it the most appropriate choice.

.