Export to markdown does not save image references

### Bug

`document.export_to_markdown(image_mode=ImageRefMode.REFERENCED)` does not save image references
### Steps to reproduce

```
def convert_pdf_to_markdown_docling(file_path: str, output_dir: str) -> str:
"""
Convert a PDF file to Markdown using Docling.

Args:
input_path (str): Path to the input PDF file.
output_path (str): Path to the output Markdown file.

Returns:
str: The markdown content
"""
IMAGE_RESOLUTION_SCALE=2.0
pipeline_options = PdfPipelineOptions(do_ocr=False)
pipeline_options.images_scale = IMAGE_RESOLUTION_SCALE
pipeline_options.generate_page_images = True
pipeline_options.generate_picture_images = True

doc_converter = DocumentConverter(
format_options={
InputFormat.PDF: PdfFormatOption(pipeline_options=pipeline_options)
}
)

conv_res = doc_converter.convert(file_path)
return conv_res.document.export_to_markdown(image_mode=ImageRefMode.REFERENCED)
```

The save_to_markdown() with the image_mode=ImageRefMode.REFERENCED works, but i am trying to keep it consistent to my codebase by returning the markdown

### Docling version

```
Docling version: 2.31.0
Docling Core version: 2.30.0
Docling IBM Models version: 3.4.3
Docling Parse version: 4.0.1
Python: cpython-311 (3.11.3)
Platform: Linux-6.10.14-linuxkit-x86_64-with-glibc2.36
```

### Python version

Python 3.11.3