
the full letter should be 512 x 376 if its not the action file will resize it to that size

create a folder called wiiletter in the root of your C: drive

open your pic in photoshop 
load the included action file wii letter.atn
run the letter action

copy the files created in c:\wiiletter\ to the \main_menu\letter\ folder of your base pack

or if you don't have photoshop paste your image to \theme helpers\wiiletter and rename it to image.png and double click on wiiletter_split.bat
and it will resize your image to the proper resolution (keeping your original file) and split your image, then you move the letter files to \main_menu\letter\

wiiletter_merge.bat will rebuild your split images and output it to image_merge.png