horizontal alignmentsubfloats
If I use subfigures to align two images side-by-side, these are right aligned. I tried several things but doesn't change anything. My current markup looks like this:
\usepackage{subfigure}
...
\begin{figure}[htbp]
\centering
\null\hfill
\subfigure[Lego Mindstorms Color Sensor 1]{\rule{4cm}{4cm}}
\hfill
\subfigure[Lego Mindstorms Color Sensor 2]{\rule{4cm}{4cm}}
\hfill\null
\rule{35em}{0.5pt}
\caption{Color / RGB Sensors}
\label{img:colorSensor}
\end{figure}
The problem with that solution using \vbox without extra precautions is that the box produced by \vbox will have a width equal to \textwidth; the second subfigure is "pushed to the right" by the first subfigure and this results in an overfull \hbox (you should see the warning message in the output console).
Now that the question has been edited to mention this is for the beamer class. here's an option using \subcaptionbox and some minipages; the height of the bigger image is measured and used as the common height for both minipages (if no captions are required, the same principle can be applied, without \subcaptionbox):
\documentclass{beamer}
\usepackage{subcaption}
\newsavebox\myfigbox
\savebox\myfigbox{\includegraphics[height=6cm,width=3cm]{example-image-a}}
\newlength\FigHt
\settoheight\FigHt{\usebox\myfigbox}
\begin{document}
\begin{frame}
\begin{figure}
\subcaptionbox{left subfigure}{%
\begin{minipage}[c][\FigHt][c]{4cm}
\centering
\usebox\myfigbox
\end{minipage}%
}
\subcaptionbox{right subfigure}{%
\begin{minipage}[c][\FigHt][c]{4cm}
\centering
\includegraphics[height=3cm,width=3cm]{example-image-a}
\end{minipage}%
}
\end{figure}
\end{frame}
\end{document}
Changing the alignment options for the minipages one can easily achieve top, bottom alignment.
In cases like this, with the standard classes, the powerful floatrow package makes the job trivial, using its heightadjust and valign keys:
\documentclass{article}
\usepackage{subcaption}
\usepackage{floatrow}
\usepackage{graphicx}
\begin{document}
\thisfloatsetup{heightadjust=all,valign=t}
\begin{figure}
\begin{subfloatrow}
\ffigbox[\dimexpr\FBwidth+4cm\relax]
{\includegraphics[width=3cm,height=5cm]{example-image-b}}
{\caption{Left subfigure}\label{sfig:testa}}%
\ffigbox[\FBwidth]
{\caption{Right subfigure}\label{sfig:testb}}
{\includegraphics[width=3cm,height=2cm]{example-image-a}}
\end{subfloatrow}
\end{figure}
\thisfloatsetup{heightadjust=all,valign=c}
\begin{figure}
\begin{subfloatrow}
\ffigbox[\dimexpr\FBwidth+4cm\relax]
{\includegraphics[width=3cm,height=5cm]{example-image-b}}
{\caption{Left subfigure}\label{sfig:testc}}%
\ffigbox[\FBwidth]
{\caption{Right subfigure}\label{sfig:testd}}
{\includegraphics[width=3cm,height=2cm]{example-image-a}}
\end{subfloatrow}
\end{figure}
\thisfloatsetup{heightadjust=all,valign=b}
\begin{figure}
\begin{subfloatrow}
\ffigbox[\dimexpr\FBwidth+4cm\relax]
{\includegraphics[width=3cm,height=5cm]{example-image-b}}
{\caption{Left subfigure}\label{sfig:teste}}%
\ffigbox[\FBwidth]
{\caption{Right subfigure}\label{sfig:testf}}
{\includegraphics[width=3cm,height=2cm]{example-image-a}}
\end{subfloatrow}
\end{figure}
\end{document}
I would use the subcaption package and \phantomsubcaption (or \phantomcaption if you decide to use e.g., subfigure environment). Note that you need version 1.1 of the subcaption package.
Below is your adapted MME:
\documentclass{memoir}
\usepackage{tikz,siunitx,mwe}
\usepackage{subcaption}
\newsubfloat{figure}
\newcommand{\scalebarimg}[6]{
\begin{tikzpicture}
\draw node[name=micrograph] {\includegraphics[width=#2\textwidth]{#1}}; %I fetch the image
\draw[ultra thick,#6] (micrograph.south west)++(0.03*0#2\textwidth,0.035*0#2\textwidth)--++(#2*#3\textwidth,0)node[above,midway]
{#4 \si{\micro\meter}}; %I draw the scalebar
\draw (micrograph.north west) node[anchor=north west,yshift=-1,#6]{\textbf{\small{#5}}}; %I draw the image label
\end{tikzpicture}
}
\begin{document}
\begin{figure}
{\phantomsubcaption\label{fig:one}
\scalebarimg{example-image}{.45}{0.33933}{50}{\subref{fig:one}}{black}}
{\phantomsubcaption\label{fig:two}
\scalebarimg{example-image}{.45}{0.33933}{50}{\subref{fig:two}}{white}}
\caption{Figures (a) and (b) should not be doubly labeled.}
\end{figure}
See Figure~\ref{fig:one} and \subcaptionref{fig:two}.
\end{document}
I surrounded each subfigure with just {} (needed to separate figure from subfigure) but you can put them for example in minipage, subfigure etc. If you want to customise label, see subcaption documentation.
Best Answer
If your aim is to equalize the spaces, then
\null\hfillis not the right way. Note that\nulldoes not start horizontal mode (that is, a paragraph), so all it does is to create some vertical space.Note also that
subfigurehas been obsolete and deprecated for 15 years.