diff --git a/docling/backend/mets_gbs_backend.py b/docling/backend/mets_gbs_backend.py
index b3ca2fb2..09ac84ae 100644
--- a/docling/backend/mets_gbs_backend.py
+++ b/docling/backend/mets_gbs_backend.py
@@ -22,7 +22,7 @@ from PIL import Image
 from PIL.Image import Image as PILImage
 
 from docling.backend.abstract_backend import PaginatedDocumentBackend
-from docling.backend.pdf_backend import PdfPageBackend
+from docling.backend.pdf_backend import PdfDocumentBackend, PdfPageBackend
 from docling.datamodel.base_models import InputFormat
 
 if TYPE_CHECKING:
@@ -194,7 +194,7 @@ def _extract_confidence(title_str) -> float:
     return 1
 
 
-class MetsGbsDocumentBackend(PaginatedDocumentBackend):
+class MetsGbsDocumentBackend(PdfDocumentBackend):
     def __init__(self, in_doc: "InputDocument", path_or_stream: Union[BytesIO, Path]):
         super().__init__(in_doc, path_or_stream)
 
diff --git a/docling/backend/pdf_backend.py b/docling/backend/pdf_backend.py
index 1b0d612e..e14b147a 100644
--- a/docling/backend/pdf_backend.py
+++ b/docling/backend/pdf_backend.py
@@ -84,9 +84,9 @@ class PdfDocumentBackend(PaginatedDocumentBackend):
 
                 buf.seek(0)
                 self.path_or_stream = buf
-            else:
+            elif self.input_format not in self.supported_formats():
                 raise RuntimeError(
-                    f"Incompatible file format {self.input_format} was passed to a PdfDocumentBackend."
+                    f"Incompatible file format {self.input_format} was passed to a PdfDocumentBackend. Valid format are {','.join(self.supported_formats())}."
                 )
 
     @abstractmethod
@@ -99,7 +99,7 @@ class PdfDocumentBackend(PaginatedDocumentBackend):
 
     @classmethod
     def supported_formats(cls) -> Set[InputFormat]:
-        return {InputFormat.PDF}
+        return {InputFormat.PDF, InputFormat.IMAGE}
 
     @classmethod
     def supports_pagination(cls) -> bool: