I think the point of image is to point out that Militant Christians and Muslims are violent while Atheists are not. Let's examine the term "militant".
mil·i·tant[mil-i-tuhnt] Show IPA
1. vigorously active and aggressive, especially in support of a cause: militant reformers.
2. engaged in warfare; fighting. link
So does this make sense the way the word is being applied. Let's grant that for a moment and ask the question "Is the picture of the 'militant atheist' the only picture that would suffice? Why not one of these?"
Now, let's ask about the so-called "Militant Christian"? Anyone who would pray for or take joy in the death of anyone can't be a consistent Christian. They ain't following God!
21 “But if a wicked person turns away from all the sins they have committed and keeps all my decrees and does what is just and right, that person will surely live; they will not die. 22 None of the offenses they have committed will be remembered against them. Because of the righteous things they have done, they will live. 23 Do I take any pleasure in the death of the wicked? declares the Sovereign LORD. Rather, am I not pleased when they turn from their ways and live?
24 “But if a righteous person turns from their righteousness and commits sin and does the same detestable things the wicked person does, will they live? None of the righteous things that person has done will be remembered. Because of the unfaithfulness they are guilty of and because of the sins they have committed, they will die.- Ezekiel 18:21-24
The picture doesn't work.
Debunking Christianity: Militant Atheist?