What is logical form? (TLP 2.171–2.225)

A picture can depict only that which form it has. Sculptures depict the spatial aspect of things, paintings their colouring aspect. (2.171)
What seems presupposed as a necessary condition is the sharing of the logical form between the picture and the depicted. (2.18, 2.2?)
Tell me: what is the logical form of a cup of tea half-filled with lemon juice? (Discussion: I think “logical form” is an intolerable abstraction that is not doing any work here).

The picture cannot depict this isomorphism; it shows it. (2.172)
I know there is debate on the relevance of notions such as “similarity” as explanans of depiction, but doesn’t “isomorphism” equally beg the question?
But this isomorphism isn’t sufficient for the picture to depict the thing or event, as it requires an intention to depict, next to an incidental resemblance. (Discussion)

One cannot see from the picture whether what it depicts exists or not. (2.224) To find that out we would have to compare the picture with the real. (2.223)
I like that, but it is not always clear what “comparing” would be like. Surely, we know to be looking at a picture if we see one, i.e. before we have made a comparison with whatever it depicts? I would put it like this: Pictures are instances of a certain procedure; that procedure is based for its constituencies on instances where comparing the picture to what is depicted would show us how pictures work: to wit: by resemblance, isomorphism.
If one were to define every individual picture in terms of the isomorphism it has with states of affairs in the world, that would count as an ontological fallacy. The definition is of pictures generally: “of the procedure”. (Discussion)

A picture suggests to its viewer what the world would have to be like if the picture depicted it. A picture merely presents us (not: “represents”) a possible state of affairs. (2.201, 2.225)

Can I make sense of the notion of logical form? I think logical form is an abstraction of a vastly different kind then a picture is of its depicted, or a sculpture is of its model. (Discussion)