They won't be sitting staring like someone at a desktop. They may be holding the phone with one hand and doing several other things at the same time. You will need alternative text in any case. Many phones don't take graphics. You can take in an image at a glance, without squinting at lines of text.
Pictures stick in the memory.
Symbols take up very much less space than words. I produced a cartoonists' monthly for two years, and was forever trying to squeeze just one more cartoon into a crowded page. Pictures stayed readable even at a fraction of their size. Captions were a different story. They had to be removed and enlarged again.