for loop in frame subtitle of beamer

I would like to build multiple slides of a frame in beamer by using a for-loop to load a specific image file out of a group in each slide.

Currently, I can either loop inside the frame and advance in slides:

documentclass{beamer}
usepackage{tikz}
begin{document}
begin{frame}
    frametitle{A title}
    framesubtitle{A subtitle}
    foreach i in {1,2}{% loop 
    only<i>{includegraphics[height=0.3textheight]{pici.PNG}
    This is pic i.}
    }% end of loop
end{frame}

Or I can iteratively create different frames, leaving the same title but advancing the subtitle:

documentclass{beamer}
usepackage{tikz}
begin{document}
foreach i in {1,2}{% loop 
begin{frame}
    frametitle{A title}
    framesubtitle{A subtitle for pic i}
    includegraphics[height=0.3textheight]{pici.PNG}
end{frame}
}% end of loop
end{document}

The last outputs the result I am after, but it is only a workaround: frames are separate, but simply with redundant title.

When I try to loop inside the frame including only the subtitle, like below, it doesn’t work:

begin{frame}
   foreach i in {1,2}{% loop 
      frametitle{A title}
      framesubtitle{A subtitle for pic i{} that doesn't increment}
      includegraphics[height=0.3textheight]{pici.PNG}
   }% end of loop
end{frame}

Any ideas how one may achieve this? The advantage of this is that one can add a brief incrementing description in the subtitle, instead of the main text, without taking space thus from the textheight available for the image(s).

Edit:
Also tried defining an array and passing this as the argument, but still didn’t work:

documentclass{beamer}
usepackage{mwe}  % added based on Andrew's comment
usepackage{tikz}
usepackage{arrayjobx}
%
begin{document}
newarraymyIncStr
readarray{myIncStr}{1&2}
begin{frame}
foreach iiChar in {1/a,2/b}{% loop 
    frametitle{A title}
    framesubtitle<i>{Subtitle myIncStr(i)}
    only<i>{%
        includegraphics[height=0.3textheight]{example-image-iChar}
        
        myIncStr(i)%
    }
}% end of loop
end{frame}
end{document}

The only way I succeeded in the subtitle’s update was by using a counter in its argument. Note how its evaluation leads +1 the slide’s main text.

documentclass{beamer}
usepackage{mwe}
usepackage{tikz}
newcounter{acounter}
begin{document}
setcounter{acounter}{0}
begin{frame}
foreach iiChar in {1/a,2/b}{% loop 
    frametitle{A title}
    framesubtitle<i>{Subtitle theacounter{}} % evaluated after stepping up
    only<i>{%
        includegraphics[height=0.3textheight]{example-image-iChar}
        
        theacounter{} 
        stepcounter{acounter}
    }
}% end of loop
end{frame}
end{document}

documentclass{beamer}
usepackage{mwe}
usepackage{pgffor}

begin{document}

begin{frame}{A title}
   framesubtitle<1>{A subtitle for pic 1 that doesn't increment}
   framesubtitle<2>{A subtitle for pic 2 that doesn't increment}
   foreach img in {a,b}{% loop
      includegraphics<sli>[height=0.3textheight]{example-image-img}
   }% end of loop
end{frame}

end{document}

documentclass{beamer}
usepackage{mwe}
usepackage{pgffor}

begin{document}

begin{frame}{A title}
   foreach ind/img in {1/a,2/b}{% loop
      framesubtitle<ind>{A subtitle for pic ind{} that doesn't increment}
      includegraphics<ind>[height=0.3textheight]{example-image-img}
   }% end of loop
end{frame}

end{document}