Maybe I did something wrong, but I tried to define a function I think as follows:
(defun save () (save-image))
— and I think my microcontroller immediately saved an image (i.e. right upon the definition, without calling it explicitly). Enabled was the editor and SD card support on Teensy 4.1.
This does not matter all too much in practice, I guess, but I thought I should mention it, in case it indicates other issues.