How do I set the default 'save image as' name for an image generated in PHP?

I have a page in my site, displaying some images that are produced my PHP. When I right click on an image and click Save Image As I get as default name the name of the php file used for generating the image.

This is for example the html for the image :

 <img src="picture_generator.php?image_id=5&extension=.png">

and the name I get is: picture_generator.php.png

Is there a way to set this name to a default one?

Thanks in advance