bfconvert: fix conversion of individual series

This ensures that the image width and height are set correctly if only
one series is being converted.  Previously, attempting to convert a
series with dimensions different from series 0 would have resulted in a
FormatException.
Merge pull request #736 from melissalinkert/rebased/develop/convert-series-fix

bfconvert: fix conversion of individual series (rebased onto develop)