It's possible that this answer is late, but if you want the user to print just the picture, then I think the best approach is to:
Open a new page in a new window/tab which only contains the image (just the <img>
tag in the <body>
tag). This way the user can select print from the menu and just print the picture.
This way it is clear for the user what gets printed.
Also you could pop up a print dialog with window.print which is not in specifications but will work in all the desktop browsers regardless.
Some clarification:
I know about print styles, and that you can completely reorder your page with them, but I found that a user actually expects the same layout to be printed what she/he sees on the monitor. By making and showing a new page just for printing the user gets a "preview" of what will get printed and therefore he will be more confident to print it.