ImageBlock should be ImageElement.

Using ImageBlock will cause the Slack API to fail:

The server responded with: {'ok': False, 'error': 'cannot_parse_attachment'})

While using ImageElement will cause a mypy error:

error: List item 0 has incompatible type "ImageElement"; expected "Union[Dict[Any, Any], ImageBlock, TextObject]"

The Slack SDK version

slack-bolt==1.7.0
slack-sdk==3.9.1