Above: Example of a macro meme photo-text composition.
An Internet viral photo in the format of a "macro meme" is typically a photo-image with a text statement overlaid across it, very often two lines of text, one above, and one below the photo. (In this regard the meme-macro may be contrasted and compared with classical photo-text works-of-art by such creators as, say, Victor Burgin or Ken Lum.)
The meme-macro becomes popular by a process which parallels the transmission of an infectious virus such as rhinovirus (the common cold) or influenza, etc. Viral pathogens are typically "caught" by contact with infected droplets--these might be airborne (the residue of an infected person's cough or sneeze); reside on a contaminated surface (known as fomites, e.g., a door-handle, a cooking utensil); or passed from-one-person-to-another by touching (hand-to-hand or sexual contact, etc.). The key parallel between the meme photo and the pathogen is the mode-of-transmission: the virus aims to gain a host (entry), reproduce, and then spread to further hosts (known medically as "shedding"). And equally, the Internet meme must also cause the viewer/recipient to become aware of it visually (entry), to copy it (reproduction), and then pass it on to other Internet or social media users (shedding). The function of the content of the meme photo then has a single fundamental core role--and by which it can be judged, without recourse to any aesthetic criteria: it must simply be amusing enough or silly enough that a recipient is inclined to share it with another user.
Beyond the parallels in transmission-reproduction, perhaps the most significant parallel between the viral pathogen and the viral Internet meme is purposelessness: a virus has no known purpose whatsoever beyond reproduction itself. And so too, the viral photo has no value or purpose (as opposed to, say, a shared news photograph). A photo-based meme that authentically "goes viral" is (like a pathogen) rather pointless--transmission itself is the sole criteria, aim or goal.
(7 February 2018)