Multiple image field in block form
The solution is an image field configured in a custom block type.

Block forms are configuration and don't support image fields. Custom block types are content and they support the same field types as nodes.

If you can't do what you want with a custom block type, you can create your own block plugin which uses a block content entity.

For background information see these questions Adding custom block problems and  Blocks: Old vs. &quot;Custom&quot; and Config vs. Fields.