Center for Bioethical Reform. Even their home page has a graphic video (with a 7 second warning so you can stop it if you want, but you actually have to take action not to see the video). Their GAP (Genocide Awareness Program) brings a large display with graphic images to college campuses.
Operation Rescue has buses and trucks plastered with graphic abortion images. This makes it harder to avoid seeing them, for better or worse. On the other end of the spectrum are sites like Priests for Life, which have graphic abortion photos on the site but you have to look for them. Or the eponymous 100 Abortion Pictures, whose name indicates what you will see if you visit it.
I know different people have different tolerances for them, as shown in these videos.
But do they work? I have heard people say that at functions like Planned Parenthood fund raisers they are very effective at dissuading donations. In the news media, I don't think any main stream media showed pictures of the horrors of the Kermit Gosnell abortion clinic, even though they were available to the public.
The GAP claims to be effective on campuses, but I know I for one will not support the Center for Bioethical Reform because last year they picketed 49 Catholic churches with graphic abortion images because the bishop of the diocese had been critical of Fr. Frank Pavone, head of Priests for Life. While I want to see abortion end, and I am not personally against the use of graphic images, disrupting masses to "punish" Catholics who live in a diocese whose bishop you don't like is neither useful nor an appropriate use of the images.
The fact is, for all the rhetoric on both sides, nobody to my knowledge has numbers to say how effective is the use of graphic images outside abortion clinics. I have only my own anecdotes to add to the confusion. At the clinic where I go each week, one of the gentlemen recently acquired a grisly image of part of a baby's head being held with forceps. The pictures is approximately five feet square, and he holds it on the side of sidewalk facing the clinic, so it is really only visible to people going in and out of the clinic. One day a woman came up to him and repeatedly kicked at the picture until she broke the foam panel that holds it. Another time a man came up and argued with us about it for several minutes, before agreeing that it was a free speech issue and he was in the wrong. But several women have come out of the clinic, walked up to him as he held this image, and thanked him for it, saying they changed their minds because of it.