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 getDom() runs, 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;