A function has arguments as inputs, and it is up to the function to interpret these arguments as appropriate. Thus, you would declare your function as:
function output = function_name(jpegfile)
%check that file is jpeg:
try
info = imfinfo(jpegfile);
catch
error('File not found or not an image');
end
assert(strcmp(info.Format, 'jpg'), 'Image is not jpeg');
%do something with file...
And you specify the file in the call to the function
result = function_name('C:\path\photo.jpg');
Unless, you meant to have the file a constant in the function, in which case:
function output = function_name()
jpegfile = 'C:\path\photo.jpg';
%do something with jpegfile
