Upload and get images in custom module with custom block form
In Drupal there is not much help available to store images in configuration. You would need to do most things yourself, like build the form fields, submit and store the images and take care how they should be rendered.

But Drupal has a great infrastructure to store images in content. So the better approach is probably not to use a block plugin, but to create a custom block type, which is content like a node. Here you can add the image fields  and configure how they should be rendered in ui.