'Save file as...' button

Hi there, is there a script which, when pressing a button, makes the ‘Save file as…’ or ‘Save link target as…’ menu appear? Usually an image has to be loaded into a screen and the ‘Save page as…’ / ‘Save image as…’ selected in the dropdown menu of your browser, but I need a button which prompts the same screen and have a particular jpg on the server selected for downloading. Any help MUCH appreciated!!