Since the game has several status-screens (like WorldMap, Item-Selection, Overlay-Status etc.) each screen has its own ARC archive.
The "Scrn" directory inside the ARC archive contains BLO files. These files contains information about the position of the images on the screen (yes, the screen-layout). For each image the files simply contains the filename, the offset on the screen, the rotation, the scalingfactor and maybe some other effects.
The "Timg" directory contains the images itself (the BTI files). These are the files you are looking for. You can convert them with a BTI image-decoder to a Windows readable format.
Sometimes the files are compressed with the Yaz0 compressor. In some archives the complete archive (ARC file) is compressed and in other archives the BTI-files are individually compressed. If the file is compressed with the Yaz0 compressor, the first 4 bytes in the file (look with a hex-editor) are "Yaz0".
Most of the images of the user-interface are stored in de directory:
"The legend of zelda - the wind waker.gcm\Res\Msg"
The ItemIcons are stored in the archive:
"The legend of zelda - the wind waker.gcm\Res\Msg\Itemicon.arc\Timg"
The on-screen icons (like Life-Status, Button-Images etc.) are stored in:
"The legend of zelda - the wind waker.gcm\Res\Msg\Menures.arc\Timg"