Хорошо, разобрался, и вы можете дать мне знать, если что-то выглядит глупо
//Create a transform
TransformedBitmap tBmp = new TransformedBitmap();
tBmp.BeginInit();
//Set the source = to the image currently selected
tBmp.Source = _Scans[_selectedImage].MyImage;
RotateTransform rt = new RotateTransform(180);
tBmp.Transform = rt;
tBmp.EndInit();
//Create a new source after the transform
BitmapSource s1 = tBmp;
BitmapImage bi = BitmapSourceToBitmapImage(s1);
//Add create the item and replace the current item in the collection
//edited according to comment
//ScannedImages s = new ScannedImages();
//s.MyImage = bi;
//_Scans[_selectedImage] = s;
Scans[_selectedImage].MyImage = BitmapSourceToBitmapImage(s1);