You are passing a string to imshow, which it interprets as a filename. However, there isn't a file called "img1". If you wish to read all the images into main memory (MATLAB workspace) and then process them, it's best to use a cell array, like this:
img{1}=imread('30.jpg');
img{2}=imread('25.jpg');
img{3}=imread('20.jpg');
for i=1:3
figure;
imshow(img{i});
end
Alternatively you can store the file names as a cell array of strings and read them in one at a time, just before displaying them:
imfile{1}='30.jpg';
imfile{2}='25.jpg';
imfile{3}='20.jpg';
for i=1:3
img = imread(imfile{i});
% process img here if you wish
figure;
imshow(img);
end
I recommend revising MATLAB strings and array types - the introductory documentation is very helpful.
