Well, Aldrea explains in the Hork-Bajir Chronicles that thought-speak works not only in words but also in pictures and symbols anyway. That's the main reason why everyone can understand it.
The way I figure, the author just transcribes thought-speak into words because that's the only way that it can make sense to the readers. And maybe the Animorphs' minds automatically put the thought-speak into words, perhaps without even realizing it, so that they didn't see it as a series of pictures except when they run into things (like when Elfangor sent them pictures of Yeerks) that don't have associated words at the time. It's also possible that Elfangor would 'think' in English (due to his time spent previously on earth), which is why that one picture in particular stood out.
As for 'thinking' in pictures having practical purposes, I can't think of a time in the books when they would have really been able to use that. Maybe in #49, when they were trying to guide Loren around? I can't think of anything else.
And it may also be true, as some have already pointed out, that young Andalites and humans can't send detailed pictures like that.
Anyway, in summary, I don't think it was a KASU at all.