It depends on the type of image you receive. As you mentioned, it might have been an image of child pornography. In that case, it's clear. However, it could be another type of image in a situation where the victim might face a form of extortion. The image may seem harmless to most people, but for that person, it has an impact.
Again, we have to ask ourselves whether this is really a crime. The sender sent an image, but there may not be any message attached to it. Furthermore, we do not know the purpose behind sending this image. We must determine whether, legally, there is an intent to commit a crime behind this action.