You need to first load your mat file with the load() function
if exist('image.mat', 'file')
% Load image from mat file.
storedStructure = load('image.mat');
% Get the image.
yourImage = storedStructure.yourImage;
% yourImage is whatever you called your image when you saved your mat file.
% Display it.
imagesc(yourImage);
% Get rid of the goofy default colormap
colormap(gray(256));
else
msgbox('File does not exist.');
end