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}
Here's a flexible solution; in a figure environment you define with \xsubfigure the objects you need and then arrange them as you like. Each command has as a first argument a symbolic key to be used in \makerow; the second argument defines the objects via a key-value syntax (add \label in the body of caption, if needed).
\documentclass{article}
\usepackage[font=footnotesize]{subcaption}
\usepackage[demo]{graphicx}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\xsubfigure}{ m m }
{% #1 is a symbolic key, #2 is a list of key-value pairs
\roly_xsubfigure:nn { #1 } { #2 }
}
\NewDocumentCommand{\makerow}{ m }
{% #1 is a list of symbolic keys
\roly_makerow:n { #1 }
}
% define the keys
\keys_define:nn { roly/subfigures }
{
width .tl_set:N = \l_roly_subfig_width_tl,
body .tl_set:N = \l_roly_subfig_body_tl,
caption .tl_set:N = \l_roly_subfig_caption_tl,
}
% the needed variables
\dim_new:N \l_roly_row_height_dim
\box_new:N \l_roly_body_box
% this is the inner command that stores the properties
\cs_new_protected:Npn \roly_xsubfigure:nn #1 #2
{
\prop_if_exist:cTF { l_roly_subfig_#1_prop }
{
\prop_clear:c { l_roly_subfig_#1_prop }
}
{
\prop_new:c { l_roly_subfig_#1_prop }
}
\keys_set:nn { roly/subfigures } { #2 }
\prop_put:cnV { l_roly_subfig_#1_prop } { width } \l_roly_subfig_width_tl
\prop_put:cnV { l_roly_subfig_#1_prop } { body } \l_roly_subfig_body_tl
\prop_put:cnV { l_roly_subfig_#1_prop } { caption } \l_roly_subfig_caption_tl
}
% this is the inner command for producing a row
\cs_new_protected:Npn \roly_makerow:n #1
{
% get the heights of the objects on a row
\dim_zero:N \l_roly_row_height_dim
\clist_map_inline:nn { #1 }
{
\hbox_set:Nn \l_roly_body_box
{
\prop_item:cn { l_roly_subfig_##1_prop } { body }
}
\dim_compare:nT { \box_ht:N \l_roly_body_box > \l_roly_row_height_dim }
{
\dim_set:Nn \l_roly_row_height_dim { \box_ht:N \l_roly_body_box }
}
}
% produce a line
\clist_map_inline:nn { #1 }
{
% a subfigure is set here
\begin{subfigure}[t]{ \prop_item:cn { l_roly_subfig_##1_prop } { width } }
\raggedright
\vspace{0pt} % for top alignment
% the body is set in a suitably dimensioned parbox
\parbox[t][\l_roly_row_height_dim]{\textwidth}{
\prop_item:cn { l_roly_subfig_##1_prop } { body }
}
% add the caption
\caption{ \prop_get:cn { l_roly_subfig_##1_prop } { caption } }
\end{subfigure}
\hspace{2em} % some space between the objects in a row
}
\unskip\\ % end up the row
}
\ExplSyntaxOff
\begin{document}
\begin{figure}
\captionsetup[subfigure]{justification=justified,singlelinecheck=false}
\centering
\xsubfigure{A}{
width=0.3\textwidth,
body={\includegraphics[width=3cm,height=2cm]{fig/workflow-S}},
caption={workflow $S$ and some text added to go to the next line}
}
\xsubfigure{B}{
width=0.5\textwidth,
body={\includegraphics[width=6cm,height=2.5cm]{fig/workflow-S-run}},
caption={run of $S$}
}
\xsubfigure{C}{
width=0.3\textwidth,
body={\includegraphics[width=2cm,height=1.5cm]{fig/workflow-S'}},
caption={workflow $S'$}
}
\xsubfigure{D}{
width=0.5\textwidth,
body={\includegraphics[width=4cm,height=1cm]{fig/workflow-S'-run}},
caption={run of $S'$}
}
\makerow{A,B}
\medskip
\makerow{C,D}
\caption{Here's a figure}
\end{figure}
\end{document}
Note that if you're not satisfied with the arrangement, you can rearrange the subfigures by saying, for instance,
\makerow{A,D}
\medskip
\makerow{C,B}
In the example I set height and width for every \includegraphics, as I don't have your images, but you can use the keys you prefer.
Best Answer
You have two options:
Omit the space between the two subfigures putting a
%symbol after of the first\end{subfigure}(i.e\end{subfigure}%).Reduce the width of the subfigures for example to
0.45\linewidth, (i.e.\begin{subfigure}{.45\linewidth}). This allows to keep the space between both subfigures without forcing a change of line.