You can't really. Browsers will create a Save as dialog box on their own when they receive a file type that they don't know what to do with.
If you want them to save the image instead of displaying inside the browser, you can zip the image, and then deliver that as a link.
For example:
Code:
<a href="imagefile.zip"><img src="imagefile.jpg"></a>
This will create an image, that can be clicked. when it is clicked, the browser will get the zip file, and automatically by itself offer the option to save it.
However if you were to change the link ref. from imagefile.zip to imagefile.jpg. the browser will just open it up, without offering the Save as option, because it knows what the type is and what to do with it.
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.