All of the files pertaining to the title screen will be inside "Title.szs" and "Title_?*"
*? being the language (i.e. Title_U for English, Title_J for Japanese, etc.), because they're mostly used for text files. All of the names can be found here
The location of the images is inside the "title" folder
> Title.szs / Title_ : . : title : timg
All of the picture names that appear as the title screen start with "tt_title_screen_..."
for example - > Title.szs / Title_ : . : title : timg : tt_title_screen_luigi.tpl
The file names that have "bokeboke" on the end are the blurred versions that appear once you press A (behind the licences). You can choose whether or not to apply this to your photos, It wont make the game crash or anything, It's just an extra step.
One thing to note though is that your whole image should not go into the files inside "Title.szs". The reason they look cut off is because they are. All of the various menus that originally came with the game (i.e. peach, bowser, mario, etc.) might look different, but the top always stays the same. So, Nintendo cut off the top and made it a separate file inside "Title_U.szs" (tt-title_screen_title_rogo_r_only.tpl). Therefore, you wont be able to have various pictures look correct unless they all have the same top.
I suggest using one photo and replacing all of them. To achieve a image that looks flawless (top image lines up with bottom), I would suggest using a slice tool where you can determine the size of the slices, because you have to keep the image the same size as the originals. I use one in Photoshop CS2.
This might take some trial and error, but I hope this helps!