Yes, I get that this is one accepted definition of agnosticism, which people do use, and you have chosen to use here. Yes, this is the definition which you are holding to be true. It's even compatible with the sense that I usually use it in (kind of) and would work perfectly fine as a modifier for the babies pictured in the image.

You originally posted that the term 'agnostic' for this image should have been used, instead of the term 'atheist', as the former would refer more acurratelly to the image posted. This is under the premise that the term atheist refers to an active state of disbelief in the proposition posed by theists. I assume this means (please correct me if I'm wrong) that in this case the term atheism is referring to an active disbelief in god, maybe phrased differently as a belief that god does not exist (a belief in a lack.)

Also going by Wikipedia

Atheism is, in a broad sense, the rejection of belief in the existence of deities.[1] In a narrower sense, atheism is specifically the position that there are no deities.[2][3] Most inclusively, atheism is simply the absence of belief that any deities exist.

(ala, lack of belief vs belief of lack)
It can refer to both. This ambiguity in definition is also present in agnosticism.

Agnosticism is the view that the truth values of certain claims—especially claims about the existence or non-existence of any deity, but also other religious and metaphysical claims—are unknown or unknowable.[1][2] Agnosticism can be defined in various ways, and is sometimes used to indicate doubt or a skeptical approach to questions. In some senses, agnosticism is a stance about the difference between belief and knowledge, rather than about any specific claim or belief. In the popular sense, an agnostic is someone who neither believes nor disbelieves in the existence of a deity or deities, whereas a theist and an atheist believe and disbelieve, respectively. In the strict sense, however, agnosticism is the view that human reason is incapable of providing sufficient rational grounds to justify the belief that deities either do or do not exist. Within agnosticism there are agnostic atheists (who do not believe any deity exists, but do not deny it as a possibility) and agnostic theists (who believe a deity exists but do not claim it as personal knowledge).

(I'm only using Wikipedia because you suggested it. Feel free to suggest a different reference if you so feel inclined.)

In this case for agnosticism, it can refer to merely not having knowledge of a subject, that humans currently don't have the knowledge of a subject, that humans can't have knowledge of a subject etc. Also included in possible definitions of not having a position of belief with respect to gods. (Minor side point, "disbelief in gods" as phrased in this article, is not always the same as believing that there are not gods, which is where I could have gotten confused, as it seemed that you were trying to contrast the agnostic viewpoint against this phrase.)

Agnosticism can refer to disbelief in gods. In fact, depending on the context, it often does. My gripe is that you would insist for atheism a definition that would not fit this particular circumstance (belief in a lack) when many people here use a valid definition themselves that would fit this circumstance (the absence of belief that any deities exist) while implying that agnosticism would fit this circumstance, when I could easily choose a definition of agnosticism that wouldn't (actual held position that we currently do not have knowledge of a subject, or more narrowly that we cannot have this knowledge.)