[memobug]

I have mine opening in a new window, but to get it centered and without scrollbars, you'd need to know the image dimensions, which would mean GD calls or putting the image dimensions in the attachments database. That would be a good thing to do at the time that the image is uploaded and the thumbnail is created.

Also such a feature does require JavaScript and might interfere with people's ad blockers. So mine has reasonable fixed dimensions, and slight offset from the corner. It falls back on a simple target=BLANK type new window if there is no Javascript. Sounds like it is not nearly what you want. But it's only a template hack which means I couldn't post it here anyway but feel free to PM me with an email address if it is something you can use.

(NOT CENTERED, NOT EXACT SIZE, YES RESIZEABLE - template mods only)

Regards,

Matt