If you have the Image Processing toolbox there is a demo for how to correct for non-uniform illumination. That might help. If you dont have that one you could try homomorphic filtering. Maybe something like this:
fK = suitable_high_pass_filter_kernel;
im_out = exp(real(ifft(log(Im_in).*fK))) + ...
mean(Im_in(:));
Or if it is "just" a question of vignetting you could try to correct your image thusly:
im_out = im_in./cos(theta).^n;
where theta is the approximate angle of the pixel lines-of-sight
HTH.
