All that's left of the original image is a generic profile of Sydney which should not require any attribution.
Wow. It's fairly disappointing seeing an attitude like that displayed by somebody involved in a Free Software project.
The subject of the work isn't really relevant. The only real issue is that you've created it based heavily on something that belongs to me. Not only is it just polite to credit the basis of your image, the licence terms under which you're allowed to alter it specifically demand you do so. If you can't abide by that, then the licence is terminated, and you're no longer permitted to use it at all.