i doubt they think using images of signs in their heads. i mean i dont usually think using letters of words in my head. usually just think the word, but that words usually associated with an image, and thats how i think deaf people think, and not only that, once they get older they might be able to learn to read peoples lip, and then be able to "emulate" how the word would sound, like by using their own customary phonetics, or even if theyll never even know what anything sounds like, they could always associate other things to the image of something, like a deaf person might think of an apple as a red succulent fruit.