imwrite required that you load the image. Here you just want to move the files using their filename. Use movefile() function. Something like this will work
myFolder = 'C:\Users\jungh\Desktop\Studienarbeit\200526_calibrationGalvo\Pictures\testdurchlauf110620(funktioniert)'; %Ordner festlegen, wo Bilder liegen
destFolder = 'C:\Users\jungh\Desktop\Studienarbeit\200526_calibrationGalvo\Pictures\testdurchlauf110620(funktioniert)\PicturesDiagram\';
filePattern = fullfile(myFolder, '*.png'); % Change to whatever pattern you need.
theFiles = dir(filePattern);
for k = length(theFiles):-1 : 1
baseFileName = theFiles(k).name;
sourceFileName = fullfile(theFiles(k).folder, baseFileName);
fprintf(1, 'Now reading %s\n', sourceFileName);
destFileName = fullfile(destFolder, baseFileName);
imwrite(sourceFileName, destFileName);
end
