It would be really helpful if you could provide is with details about the error (such as the exact error message, if it concerns all images or just some, etc.). If there's any problem there, we would certainly like to fix it.
(The same way you unregistered it, it can be registered again - when running
regsvr32 "C:\Program\Alwil Software\Avast4\ahascr.dll"
- i.e. ithout the /u argument.)
And of course, you already know how to unregister it