This is what it looks like when you hold down the right mouse button and turn the wheel
or use shortcut Ctrl + Tab

How to do?

in opera: config find Use Thumbnails in Window Cycle change the default value of "0" to "1".
Or (in Opera 9.5):
Go to Settings - Advanced - tabs - use thumbnails when switching between tabs

UPD: I removed the lock at the request of Vali, now everyone can see