Opera Preferences version 2.1
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Save To Folder=
to Opera Mail's preferences directory. See "Menu -> Help => about Opera Mail" for the location. Then, you'll get an entry to Open in the Opera web browser in the "open with" menu in the attachment.
Add more entries for other image types. You can view source of an image with an attachment to see what content-type an attachment is, so you know what one to add.
You can goto "Alt + P -> advanced -> downloads" in Opera 12, configure all the types you want to open with a specific application and check "pass web address directly". Then, apply and close Opera 12. Then, you'll have its handlers.ini that you can just copy over to Opera Mail.