If you're wanting metadata, consider exiftool also:

    $ curl -s http://4.bp.blogspot.com/-MPv_CwvvwKQ/Ulrw3TfdgyI/AAAAAAAAAEw/YsRPmU6C5xM/s1600/trefoil_rotate_white.gif | exiftool -

Output contains:

    Comment : Created by Wolfram Mathematica 9.0 for Students - Personal Use Only : www.wolfram.com