You have done almost everything correct. However, you are over-writing the excel file with each iteration, thus you will only get the values corresponding to the final file.
You can either store data for images in separate columns -
path_directory='myfolder'; % 'myfolder' in my current directory
myfiles=dir([path_directory '/*.jpg']);
%number of files
n = numel(myfiles);
%As it is known that there are 25 bins for categorizing data, the output
%will be a 25x1 vector, thus preallocate accordingly
count = zeros(25,n);
for k=1:n
filename=[path_directory '/' myfiles(k).name];
I=imread(filename);
I=rgb2gray(I);
count(:,k)=imhist(I,25);
end
T = table(count);
writetable(T, 'frequency.xlsx');
Or you can store the data for images in separate sheets -
path_directory='myfolder'; % 'myfolder' in my current directory
myfiles=dir([path_directory '/*.jpg']);
%number of files
n = numel(myfiles);
%preallocate worksheets
writetable(table(),'frequency.xlsx','Sheet', n);
for k=1:n
filename=[path_directory '/' myfiles(k).name];
I=imread(filename);
I=rgb2gray(I);
[count,x]=imhist(I,25);
T=table(count);
writetable(T, 'frequency.xlsx', 'Sheet', k);
end
