Your code does not show where 'objlistCropped is defined, but, assuming it is a List of Type 'Selected:
1. The error occurs because you are re-using the same instance of a Selected, 'objCropped, each time: 'objCropped is clearly a reference Type.
2. You can fix this, I am pretty sure, by just creating a new Instance of 'Selected to add each time you update the List:
objCropped = new Selected();
objCropped.Image = strCroppedImagename;
objlistCropped.Add(objCropped);
Of course, if you used this approach, then you could declare 'objcropped at the start of your code without initializing it.