fix: force conversion to RGB to solve runtime error

nithumb.py

@@ -16,4 +16,6 @@ image = load(parsed.inputfile).get_data()
 image = Image.fromarray(image[:, :, image.shape[2]//2] if image.ndim == 3
                         else image)
 image = image.resize([128, 128])
+if image.mode != 'RGB':
+  image = image.convert('RGB')
 image.save(parsed.thumbnail)