Use dir() to get all the possible extensions. Inside your loop over i (bad name by the way), have this code:
baseFileName = sprintf('DSCN0594%d.*', i);
filePattern = fullfile(pwd, baseFileName); % Use whatever folder you want instead of pwd.
allFiles = dir(filePattern);
for k = 1 : length(allFiles)
% Get the full name of the kth file.
thisBaseFileName = allFiles(k).name;
thisFullFileName = fullfile(pwd, thisBaseFileName);
% Read in original image.
originalImage = imread(thisFullFileName);
% Crop it and save cropped version in croppedImage.
croppedImage = imcrop(originalImage, coordinates);
% Save to current folder (which is a bad idea!) with filename
% that depends only on i (again, bad idea).
imwrite(croppedImage, [num2str(i),'.png'])
end
That's just off the top of my head, and not tested, but it should find all versions of the file regardless of whether they are PNG, JPG, TIF, BMP or whatever, and save all of them to the same filename. You might want to change the filename, or save them to the same format as they started with, or else if you have both a PNG and a JPG with the same base file name, the first one will get overwritten.
