You seem to be missing the point that the picture theory of meaning is so called because a picture is an example to illustrate his theory. A picture stands in a certain relationship to its subject (what it is a picture of). Another common example is the relationship between the written score of a piece of music and the music as played and a recording of a performance.
In a word, these are all cases of same structure. The elements of the picture (the patches of colour), when they are arranged in a certain way, represent the scene being pictured. Similarly, the elements of the score, the performance, the recording, when they are arranged in a certain way represent (or present) the music.
Once we understand that relationship we can see the picture as a picture of something else, which is quite unlike it. When we understand the relationships in the score, we can play the music and construct a technology to record it.
The general concept is complicated, but the application to language is very tempting. It starts from the truth-functional calculus, the correspondence theory of truth and the idea that some propositions are atomic, because when they are analysed into words, they lose their sense (capacity to be true or false).
I won't go further with this, because I think I've explained that the Tractatus theory in no way presupposes that language learners need to be able to see. SEP - Logical atomism
I don't know how difficult it is for a person blind from birth to learn a language. But it is probably much more difficult for someone who is deaf from birth to learn language, and for someone who is deaf and dumb more difficult still. Helen Keller (who was deaf-blind) is a famous case in point. Wikipedia - Helen Keller