Something like this?
https://addons.opera.com/en/extensions/details/images-onoff/?display=en

Yes, thank you very much

If you have viewbar enabled in Opera 12.17, there is a Show Image button that shows,hides, or cache images. Is there a feature like that in Opera 27 or any other browser out there?

Thanks

I must apologizes for my solution, such that stng 's solution is the appropriate solution. Selective masking is truly better than global masking. After all, the problem only occurs on youtube and google. I like using Opera, and hope sites would recognize me using opera rather than a Firefox mask.
I have applied stng 's solution and it worked. However, I did have a hard time finding override.ini because that's for Mac users. For windows, it's called override_downloaded.ini.

Also, where do you get these User agent strings? Is it better to use the most updated one? I tried both Mozilla/5.0 (Windows NT 5.1; rv:31.0) Gecko/20100101 Firefox/31.0 and the one stng provided, but there is no difference.

It appears some features from Google isn't compatible Opera 12.17. Google images, search layout , and the youtube comments, reverted back to the old style. There is also a few thread regarding this issue, so it's not just you! But here's the solution

Go to "opera:config --> User Prefs" and copy this line:
Mozilla/5.0 (Windows NT 5.1; rv:31.0) Gecko/20100101 Firefox/31.0
to the "Custom User-Agent" field.
Don't forget to click "Save".