i seem to be the only person on the planet who ever used Web Log Filter that it gave me a real fright to see a question about it.. definitely not one of my more popular tools.
i actually recently used Web Log Filter for the first time in a while and it took me a few tries to remember how to use it.. it's not the most obvious thing. but it does work and serve a useful purpose.
one thing that tripped me up was remembering that it doesn't want to modify the original file, but create a new output file.
can you first make sure you can figure out how to get it to process your file and create a new output file, and then experiment a bit with getting it to reject lines.. once you get that working, let me know again if you still have trouble blocking images.
And definitely take a look at the included NoImages rule carefully -- it's just a sample and it may not cover all cases, etc.