The issue will never be solved, spam on imageboards is a perpetual arms race.
In terms of what I have done to combat this in the last 16 hours, I have attempted to enabled tesseract OCR in order to see if OCR can detect the advertisting on the image and add it to the post body in order to make post filters based on that.
So far that doesn't look like it is working correctly.
I have also added specific name and subject filters based on the current dictionary word the botnet is upto, which is only a temporary measure that causes it to jump to the next word.
I have also added a specific filename filter which is also only temporarily until the botnet gets to the next non matching image.
Recent Image for frontpage has been set to 0, and will remain that way forever, unless requested otherwise, see my initial comment.
Other post filters I had in place were changed or removed because lainons complained or attempted to ignore them (minimum post length filter).