Q:

subfigure numbering in subfigure environment (subcaption package)

My new friend is the subfigure environment of the subcaption package.  Although reading the manual by Axel Sommerfeldt i did not managed to change the subfigure numbering from lower case letters to small Roman numerals. 
I need them beside pure personal preference for having the ability to have more than 24 (z) subfigures. 
Can somebody assist me? 
\documentclass[a4paper,oneside,10pt]{scrartcl}

\usepackage{graphicx}
\usepackage{caption}
\usepackage{subcaption}

%%%%%%%%%%%

\begin{document}

\begin{figure}
    \centering
    \begin{subfigure}[b]{0.3\textwidth}
            \centering
            {\rule{3cm}{2cm}}
            \caption{Picture 1}
    \end{subfigure}%
    \begin{subfigure}[b]{0.3\textwidth}
            \centering
            {\rule{3cm}{2cm}}
            \caption{Picture 2}
    \end{subfigure}
    \caption{Two picures; a black picture (i) and another black picture (ii).} 
\end{figure}

\end{document}

not desired output:

desired output:

thanks in advance
Benjamin

A:

Change the representation for the subfigure counter by redefining \thesubfigure:
\documentclass[a4paper,oneside,10pt]{scrartcl}
\usepackage{graphicx}
\usepackage{caption}
\usepackage{subcaption}

\renewcommand\thesubfigure{\roman{subfigure}}

\begin{document}

\begin{figure}
    \centering
    \begin{subfigure}[b]{0.3\textwidth}
            \centering
            {\rule{3cm}{2cm}}
            \caption{Picture 1}
             \label{sfig:a}
    \end{subfigure}%
    \begin{subfigure}[b]{0.3\textwidth}
            \centering
            {\rule{3cm}{2cm}}
            \caption{Picture 2}
             \label{sfig:b}
    \end{subfigure}
    \caption{Two picures; a black picture (\subref{sfig:a}) and another black picture (\subref{sfig:b}).} 
\end{figure}

\end{document}

