Another approach could be to temporarily cover the image with a semi-transparent shape:


    \documentclass{beamer}
    \usepackage{tikz}
    \setbeamercovered{dynamic}
    
    \begin{document}
    
    \begin{frame}
    
    \begin{figure}
    	\includegraphics[width=2cm,page=1]{example-image-duck}
    	Test A
    	\pause
    	\begin{tikzpicture}
    	\node[anchor=south west,inner sep=0] (B) at (4,0) {\includegraphics[width=2cm,page=2]{example-image-duck}};
    	\only<1>{%
    	    \fill [draw=none, fill=white, fill opacity=0.7] (B.north west) -- (B.north east) -- (B.south east) -- (B.south west) -- (B.north west) -- cycle;
    	}
    	\end{tikzpicture}
    	Test B
    	\pause	
    	 \begin{tikzpicture}
    	    \node[anchor=south west,inner sep=0] (B) at (4,0) {\includegraphics[width=2cm,page=3]{example-image-duck}};
    	    \only<1-2>{%
    	        \fill [draw=none, fill=white, fill opacity=0.7] (B.north west) -- (B.north east) -- (B.south east) -- (B.south west) -- (B.north west) -- cycle;
    	    }
    	 \end{tikzpicture}	
    	Test C
    \end{figure}
    \end{frame}
    
    \end{document}

[![enter image description here][1]][1]


  [1]: https://i.stack.imgur.com/lpjmL.gif

I've done something similar doing the following:

    \begin{figure}
                    \includegraphics<1->{problem-a.pdf}
                    \onslide<1->{Test A}                    
                    \includegraphics<2->{problem-b.pdf}
                    \onslide<1->{Test B}
                    \includegraphics<3->{problem-c.pdf}
                    \onslide<1->{Test C}
    \end{figure}

Maybe its useful to you

You can find more information about using imported graphics in pdflatex in this [document][1]. 
Look for Chapter 12: "Overlaying Two Imported Graphics".

  [1]: ftp://ftp.tex.ac.uk/tex-archive/info/epslatex.pdf

For anyone that stumbles upon this, the best thing I've found so far is to use tikz and setup a custom transparency mode:

    \gdef\transparent@value{100}
    \newcommand{\getbeamertrans}{
    	\transparent@value/100
    }
    \newcommand{\set@transparent}[1]{\gdef\transparent@value{#1}}
    \def\opaquenessCustom#1{%
    \only<1->{%
      \beamer@actions{%
	    \set@transparent{#1}%
        \expandafter\xdef\csname beamer@oldcolorhook%
        \the\beamer@coveringdepth\endcsname{\beamer@colorhook}%
        \expandafter\xdef\csname beamer@oldpgfextension%
        \the\beamer@coveringdepth\endcsname{\beamer@pgfextension}%
        {\globalcolorstrue\colorlet{beamer@freeze\the\beamer@coveringdepth}{bg}}%
        \xdef\beamer@colorhook{!#1!beamer@freeze%
          \the\beamer@coveringdepth\beamer@colorhook}%
        \gdef\beamer@pgfextension{!#1opaque}%
        \color{.}%
      }%
      {%
	    \set@transparent{100}%
        \xdef\beamer@colorhook{\csname beamer@oldcolorhook%
          \the\beamer@coveringdepth\endcsname}%
        \xdef\beamer@pgfextension{\csname beamer@oldpgfextension%
          \the\beamer@coveringdepth\endcsname}%
        \color{.}%
      }}%
    }%
    \define@key{beamer@mixin}{transparent}[15]{%
    	\def\beamer@uncoverbeforeactions{\ignorespaces\opaquenessCustom{#1}}%
    	\def\beamer@uncoverafteractions{\ignorespaces\opaquenessCustom{#1}}%
    }
    \newcommand{\BeamerGraphic}[1]{%
    	\begin{tikzpicture}%
    		{\node[opacity=\getbeamertrans] {\includegraphics{#1}};}%
    	\end{tikzpicture}%
    }