The file you'll need to open first is screens.viv, which is located in the fe folder and can be opened with the EA Graphics Editor. Extract screens~bootup~title.big and open it up with DOLFFIN. This will allow you to export the textures using the Export Blob option.
Once you've got the textures which will be in .fsh format, you can open them with FshX. FshX has an export/import function, so you can export them as a .dds file and then work on them in Photoshop or GIMP. Once you're done and saved the changes to the .dds file, it's basically a matter of reversing the process. Import the .dds into the .fsh file in FshX, import the updated .fsh file into the .big file using DOLFFIN (highlight the texture in DOLFFIN and select Import Blob).
Note that you don't have to import the .big file back into the screens.viv file. You can simply place it loose in the sgsm folder, which is the folder that the game will check for customart files before loading what's in the .viv packages. This is usually the preferable method for updating art files than reimporting into the .viv file (which can be done using the EA Graphics Editor), since you can easily delete files from the sgsm folder to restore the defaults still stored in the various .viv files.
All the programs I've mentioned can be found in our
Downloads section.