Cannot redeclare MyClass - but it's only declared once
When you are creating a custom module, you have to make sure your module registers its own namespace. In the current situation your module is attempting to use the namespace of the image module namespace Drupal\image\Plugin\ImageEffect;. Where you should actually be using a namespace of your own module namespace Drupal\mymod\Plugin\ImageEffect;

Then if you require functions of the image module, you can (as you have done already) load the other classes and functions with use. Like use Drupal\image\ConfigurableImageEffectBase;.