Modify PicturesController and associated spec for vertical resiziung and cropping

The original PR proposed to make size: '300' indicate "infer height". in this PR,
'x300' maps to "infer width" and "300x" maps to "infer height". I find this slightly
more understandable (as the "x" indicates that something's missing, and which one).

The original tests had two conflicting test cases, one where size: '10' was to be
invalid, and one where size '40' was to mean 'infer height'. This is the point where
I had to change the API for this feature.

The other thing was that in the pictures controller, there was a "c" missing in a function
that is really just used for cropping with no crop_from data.