There’s a boring definition of conversion, but I prefer to simplify it down to monkeys.
Monkey see. Monkey do.
The “do” is the conversion. But the magic is in the “see.” What does a monkey — or person — see that spurs them into doing? And what gets in the way between the “see” and the “do” that makes “doing” less likely?
Using images to improve conversion rates requires answering those two questions. Essentially, you need an image that creates a desire to take action and reduces any friction.