For an expository essay on blockchain technology, the most effective visual element that would improve the style of the presentation is a flowchart diagram.
A flowchart can clearly illustrate how blockchain works, including the process of transactions and the concept of distributed ledger technology. It helps break down complex concepts into more digestible parts, making it easier for a general audience to understand the underlying mechanics of blockchain. While color photographs, animated graphics, and decorative headings can enhance the visual appeal, they may not convey information as effectively as a flowchart in this context.