For security reasons, it's actually designed into the system that you can't just dump files into the user's directories from the web browser.
There are two options: One is to create a zip file, post it on your site, and ask the people to download the file, and unzip it in a specific location that the viewer can see. The other option is to create an .EXE file (I assume your users are Windows users if they are using some client side image viewer). They download and run the .EXE file which writes the images to the specific location.
Of course, this would only work if the users know you personally and trust that your .EXE file is safe. Even then, they would be dumb to run such an .EXE file.