The problem with your calculation it assumes that people who are likely to get converted by the first image wait around for the second image and get "unconverted".

A good intuitive way to think about it is this: if you're only showing the second carousel image to people who the first image failed to convert then there's zero down-side.

In practice there are some users who would convert if shown the first image for longer who you'll lose by rotating to another image, but that's what you need to measure to decide if it's worth having a carousel.