I think (may be wrong) the config is read only inside the module, so your line before the return won’t actually do anything to
this.config.count (you’ll probably see an error in the development tools in the browser). Every time
this.config.count is always the same, so your images will always be the same.
Instead, you need a variable you can manipulate. In your
start function, create a variable to use; e.g.,
this.photoCount = this.config.count.
Then in your
getDom() method, use
this.photoCount instead of where you have used
i and change the line before the return to
this.photoCount = (this.config.count + 1) % 2;