You can optimize your search of course, and only analyze threads/posts that contain some specific keywords. You can also narrow your database and compare with either your most recent or most pirated pictures (sets).
But that raises new issues/questions:
1. That requires much more sophisticated spidering software that filters what you need.
2. Keyword filters are bound to be innacurate, especially for mainstream/hardcore/babe sites.
3. People will still be able to post your stuff that's not in the db.
All in all, simply brute forcing all new posts and comparing then against your entire database, is the REAL solution; while everything else is bound to be less accurate and miss a lot of your stuff posted illegally.
Thus, please find us some way to make 1 billion comparisons/hour
While search optimization (for those who still want to go that route) will be better spent on adding more boards to the pool.