The image is a JPG. Most JPGs are RGB. Medfilt2() only accepts a single-channel input. You either need to make the input single-channel (gray), or you need to filter it channelwise. Alternatively, you can use medfilt3() with a single-page window specification.
% the inputs
inpict = imread('peppers.png');
winsize = [9 9];
% use medfilt2() in a loop
op1 = inpict;
for c = 1:size(inpict,3)
op1(:,:,c) = medfilt2(inpict(:,:,c),winsize,'symmetric');
end
% use medfilt3() with the window depth set to 1
op2 = medfilt3(inpict,[winsize 1],'symmetric');
% the results are identical
immse(op1,op2)
% show it
imshow(op2)
