Modernize type hints in utils/ module to Python 3.12+ syntax

## Summary
The utils module uses legacy `typing` imports that can be replaced with Python 3.12+ built-in syntax.

## File to update
- `src/blogtuner/utils/images.py`

## Current import
```python
from typing import Optional, Self
```

## Proposed change
```python
from typing import Self
```

Then replace all `Optional[X]` with `X | None` in the file.

## Priority
Medium