As it is already saved on your computer, drag and drop the image file into whatever directory your NN files are located/sources images from.
myImage = imread('myImageName.jpg'); %load the image file
grayImage = rgb2gray(myImage);
% maybe there is some processing you should do here so that the mean pixel white value for
% your image is the same as that of the MNIST data, and also centered in the window
% or maybe just test it as is and see how your NN implementation handles it
formatMNIST = imresize(greyImage,[28,28]); % change size to 28x28
% now formatMNIST should be useable in your NN