Make two figures aligned at top

Use the adjustbox package's valign=t key to align the graphics contents at the top:

\documentclass{article}
\usepackage[export]{adjustbox}% http://ctan.org/pkg/adjustbox
\begin{document}
\begin{figure}
  \centering
  \includegraphics[scale=0.5,valign=t]{example-image-a}
  \includegraphics[scale=0.3,valign=t]{example-image-b}
  \caption{A caption\label{fig:scaled_diss}}
\end{figure}
\end{document}

Using the export key with adjustbox, this will load the graphicx package, and allow you to use its keys as part of \includegraphics.

Also, for your specific example, it may be more advisable to use a fixed height, rather than a scale. This way you can more readily provide the same vertical alignment of your items (graphics or otherwise).

Since adjustbox is not easy to install on ubuntu-12.04 the solution proposed in Aligning image and text on top, with minipages can be useful.

\begin{tabular}{p{0.5\textwidth} p{0.5\textwidth}}
  \vspace{0pt} \includegraphics[width=0.49\textwidth]{example-image-a} &
  \vspace{0pt} \includegraphics[width=0.49\textwidth]{example-image-b}
\end{tabular}

A third alternative is the stackengine package, with its \belowbaseline command, which places the top of an object a given distance below the baseline, shown below without, then with \belowbaseline.

\documentclass{article}
\usepackage{stackengine}
\usepackage{graphicx}
\begin{document}

\def\figa{\rule{1in}{1in}}
\def\figb{\scalebox{.6}{\figa}}

\begin{figure}[H]
\begin{center}
\figa~\figb
\caption{\label{fig:scaled_diss}}
\end{center}
\end{figure}

\begin{figure}[H]
\begin{center}
\belowbaseline[0pt]{\figa}~\belowbaseline[0pt]{\figb}
\caption{\label{fig:scaled_diss}}
\end{center}
\end{figure}

\end{document}