dictation–can't picture words

Will dictation still work if someone cannot form a picture of words in their head? We used Spelling Wisdom last year for my then-3rd grader, and are using it again this year. I didn’t feel that it helped her spelling at all last year. It’s not unusual for her to mispell common words like for (writing it as fro) or which (writing it as wich). She wouldn’t miss those on the dictation exercises, but will miss them at other times. She’s said that she can’t picture things in her head. I can’t either. I’m an excellent speller, but I cannot form a picture of any given word in my head. I’m concerned about her poor spelling, and wondering if I should consider using something other than dictation to help her improve in this area. I do not feel she has any issues with dyslexia or vision, and she reads well above grade level.