imcrop return an image (2D or a 3D array). You cannot assign it as an element of a numeric array. You need a cell array
im = imshow('DJI_0834.jpg');
cropped = cell(1, 30);
for i = 1:30
cropped{i}=imcrop(im);
end
Access them using brace indexing
cropped{1}; % first cropped image
cropped{2}; % second cropped image
..
..
cropped{end}; % last cropped image
