The answer to your question is very simple. We live in a society where political correctness reigns supreme and has become the de facto law of the land, even taking precedence over the actual codified laws. The reason it's OK to insult Christianity but not Islam is the same reason Black Lives Matter is considered a civil rights group but the KKK is a hate group, why "gay pride" is considered civil rights activism but "straight pride" is bigoted hate speech, and so forth. If your message is left-wing, you can be as outrageous as you want, but if your message is right-wing, it's instantly cast in an evil light by the media. Despite the fact that the First Amendment is supposed to apply to all, as well as the concept of due process, the media and society as a whole has established a different ideal of what's acceptable speech, and the basic premise is "you'd better be a liberal!"
I haven't heard of anyone going to jail over disrespectful images of either religion in the United States.
The first amendment applies only to the government. It doesn't mean other people can't have an opinion about your speech. It means you can't be arrested for it. That's all it means.
And I've never heard of such a silly thing. Not to say it isn't true, I just haven't ever heard it. Then again I don't go around with images of either, disrespectful or otherwise.
Certainly wouldn't be surprising. People are pretty dumb. Many people say they like 1st amendment and think it's a wonderful thing, but then turn around and think people should be banned from saying certain things.