It's not possible to get a query for the last modified date of a variation from "image" or "imageAsset" field type.
Steps to reproduce:
1. Login as admin
2. Create a new Content Type named "MyImageAsset"
3. Add image asset fieldtype
4. Create a Content Item using that Content Type
5. Regenerate GraphQL schema and clear cache
6. Enter query similar to:
For created imageAsset:
For built-in image Content Type, that has ezimage fieldtype: