Find this on Debug Log:

{Thu, 12 Jul 2012 11:24:04 GMT}

Backtrace: USER NOTICE: Too few arguments! Image not found. in /home/hklb/public_html/hkdigit.net/pic/zp-core/functions-image.php on line 33
trigger_error called
from imageError (functions-image.php [33])
from i.php [41]

What does it mean?

That means somewhere there has been a bad call on the image processor. Since that script is always run "alone" there is no way to know where it was launched. But unless you know how it was caused most likely this is someone making up a url to it.