Interesting project. 
If all else fails, why not just make a "Screenshot Not Available" image show if the results aren't listed in the DMOZ directory? Also, are you able to resize the image on the fly like that or will it just scale the full size image down? It seems like that might already be a pretty time consuming script (having to wait on page loads from other sites). You might want to set a short timeout period and have it display the aforementioned "Image not Available" pic if it times out.
Something just occured to me...does it used cached images from the DMOZ directory? If so, that would speed things up a bit, but not make them very current.
Anyway, I'll keep my eyes and ears open about this. Good Luck.