From my understanding, subliminal messages are not possible when viewing a static piece of media.
Please question your source that provided you with such a claim and ask them what their motivation for such an unsubstantiated claim is.
Part of the problem of understanding or explaining what is meant by "subliminal message" is a certain messiness of definition. May I point out that, as a general point of reference, if some message is spoken or written or broadcast or drawn or otherwise transmitted below the threshold of perception, it cannot be received by a listener or observer and therfore will have no impact. What is meant, in the field, by "subliminal," is rather some message that is designed to be delivered in an indirect or covert way. Every newspaper, magazine, book, movie, poster, TV show, bilboard, and many works of art are filled with indirect messages. They are so pervasive, they have filled what surrounds us to become ubiquitous and form part of the fabric and complexity of our culture. Look at any ad for, say, beer and see how many "messages" are conveyed by the images. You will most likely be able to find many, including the standard message to heavy drinkers that alcohol is a cure or at least a palliative for lonliness and a "lubricant" for meeting and hooking up with the opposite sex. They may pitch the intoxicant as a "thirst-quencher" but that is just the surface message. As every art student knows, early church imagery is filled with subliminal content, from the placement of the figures against a ground to the colors used to convey different emotions. Why wouldn't a current seller of "Christian" material make use of these age-old tactics?