You DON'T want to use image as the name of a variable - it's the name of a built in function. Plus you need to convert to double when you call the convolution.
grayImage = imread('Cameraman.tif');
grayImage = double(grayImage);
subplot(2,3,1);
imshow(grayImage, []);
axis on;
title('Original Image', 'FontSize', 15);
myfilter = fspecial('gaussian',[3 3], 0.5);
subplot(2,3,2);
imshow(myfilter, []);
axis on;
title('myFilter', 'FontSize', 15);
a = imfilter(grayImage, myfilter);
subplot(2,3,3);
imshow(a, []);
axis on;
title('a', 'FontSize', 15);
b = imsharpen(grayImage,'Radius',0.5);
subplot(2,3,4);
imshow(b, []);
axis on;
title('b', 'FontSize', 15);
c=conv2(a,b, 'full');
subplot(2,3,5);
imshow(c, []);
title('c', 'FontSize', 15);
axis on;
