It also occurs to me that the Animorphs might just have never thought to send thought-speak images because they were never in a situation that required it. Elfangor himself mainly did it because he didn't have time for words.

I brought it up in fan-fiction again, but it was something that took a lot of training.

What bugged me was all the way in book #40 Mertil and Gafinilan could sense each other through thought-speak as long as they were on the same planet, but the Animorphs as far as we know never lengthened the distance through which they could communicate. Then again, we could blame that on them being human and not prone to using thought-speak most of the day.