Those .arc files have a really simple structure.
.arc file has 3 sections:
-1- Header, 16 bytes
4 bytes: Magic_ID [GARC]
4 bytes: File size
4 bytes: amount of files in the archive
4 bytes: data offset
-2- pointers, data offset - 16 bytes
it's just a bunch (amount of files, to be exact) of 4 byte pairs, the relative offset in the data section and the size of a file
-3- data, file size - data offset bytes
here, there's raw data, files as is, you can just rip one file and work with it.
Now, the sprites can be either compressed, or non-compressed.
*_eff_img.arc are uncompressed, it's a bunch of BGR555 colors.
*_img.arc are compressed. I think is't LZSS/LZ77, based on the look of it.
The main problem is, however, that there's no clear indication about sprites dimensons (width and height), i beleive they are hard-coded into the game. So it would be quite a task to get those sprites out, with some extreme romhacking skills required, not just open the file and get the images. Sorry.
