I'm willing to bet that the way that people come about these composite photos is by searching on google, or elsewhere, "picture of earth from space," or something similar. When they see the composite and recognize that Africa, for example, is huge in one and small in another, their BS meter goes off. Flat earthers, rightly so, deduce that these pictures are fake. They do a little research an come to find that NASA stated that they are composites. The problem is that they went through google image search instead of the source of the image. When a magazine, National Geographic, or Time, for example, released the original photo, they likely did have that asterisk. The caption of the photo probably said, "Composite image created by NASA [with some date]." The flat earther didn't come about the image this way though. They found it through google. They then immediately believe they are being deceived.