If you need only one image per letter, use a spritefont.
Or do you want multiple images for each letter? for example, "A" can be either an image of apple, or avocado, or ant etc. This will not be possible with a spritefont, you will have to do it with a sprite.
Create animations (A, B, C...) for each letter in the sprite. Add several images as frames into each animation. Make sure all images are the same size.
When you are displaying words, create an instance of the sprite, set animation to required letter, choose a random frame.