Late to reply to this but thought it may serve as useful for anyone hitting on this as I did from Google.
A quick and easy way to force the browser to allow you to save as a particular filename and/or extension is to pass the filename and extension into the URL (or form action depending on how you are generating the image)
eg as a form post
<form name="myform" action="picgenerator.php?name=thepic.jpg" method="post">...
or as just a URL (in the querystring)
http://www.mywebsite.com/picgenerator.php?name=thepic.jpg
Now, right-clicking on the image and choosing "Save as" (or equivalent, depending on which browser you are using) will save the file as a jpeg defaulting to the name thepic.jpg
Works for all browsers (as far as I know)
