If cropping a remote file, get dimensions from remote too. Also ensure _orig file exists

Note that using the remote file rather than a local copy has 2 problems:
1) we lose the original file (gif, png, whatever)
2) for Biopix images (which have the _580_360 version deliberately constrained to smaller then normal size), we do not preserve that constraint.
Merge pull request #116 from hyanwong/temporary_cropping_fix

Temporary workaround: If cropping a remote file, get dimensions from remote too.