As you will see elsewhere rebuilding the icon cache may work. For me, on Windows 7 64-bit, after installing Adobe Acrobat Reader DC, the icon for Opera suddenly became generic. Restarting the computer, re-pinning Opera to task bar did not help.
As mentioned by Denja there is a solution in the SevenForums: http://www.sevenforums.com/tutorials/49819-icon-cache-rebuild.html
To expand on that answer a little and protect against link rot, the SevenForum procedure involves running the following commands from a command prompt.
taskkill /IM explorer.exe /F
DEL "%localappdata%\IconCache.db" /A
shutdown /r /f /t 00
I copied the above command into notepad (you don't need to save it), closed all other programs, ran an elevated command prompt (typed 'cmd' in start menu search and then right-clicked
cmd.exe and chose 'Run as administrator'). The I copied and pasted the commands one at a time and ran them.
When executing the last command the computer will restart immediately so make sure you've saved anything you need before running the above commands.
Basically the above deletes the IconCache.db file which will automatically get rebuilt when windows starts. When you restart, your icons will be blank but will slowly repopulate within 10-20 seconds or so.
This fixed the opera icon issue for me.