It needs code. But no one will able to give u a sample code because you have not mentioned which platform are you working on.
Can you right click on the link and click save as target? Or are you looking for some code which automatically asks you to download it? Another solution may be to zip the image first?
What you need to do is make a new php page for the image called e.g. download-image.php. This page will have headers saved on it with the file name and content type attached and an "exit()" command to close the page. I dont have the sample code to hand but look up "force download php" in google and you will find your answer
a href Image will always open in a new window. if you want to download them make the image as .zip file and give that link in a href.