Ability to set image as GDM background

Closes #40.

scripts/utils/is_photo.py

@@ -0,0 +1,21 @@
+image_extensions = [
+        'jpg', 'jpeg', 'png', 'gif', 'bmp',
+        'webp', 'tiff', 'tif', 'svg', 'heic', 'heif'
+    ]
+
+def is_photo(extension: str):
+    """
+    Check if extension is supported photo format
+    """
+
+    return extension.lower() in image_extensions
+
+class NotSupportedPhotoExtension(Exception):
+    """
+    Exception for not supported photo extension
+    """
+
+    def __init__(self, extension):
+        self.message = (f"Photo extension {extension} is not supported. "
+                        f"Supported formats: {", ".join(image_extensions)}.")
+        super().__init__(self.message)