I did this project last semester. I was trying to detect copies of a master piece ‘Monalisa’ among thousands of pictures. I use two different method, one is DCT-based, another is using covariance matrix. It turns out that using covariance matrix is more robust but time consuming. Compression, rotation, resizing can all be detected. Result below is showing that using covariance matrix, we only missed one copy that modified by non-linear luminance change.