lol sorry
well i think you gonna lose what's in the internal memory anyway (songs, videos etc) so do the following:
make a new folder
paste in it the original DefaultFw.bin and BIMG and your modded AIMG
set that folder as the working folder in FRM pro
set to "program all" mode and put a tick in the "disk img mode" box. then press ISP and ure done
and trust me, i hate Sunplus frmom the bottom of my heart. because simple videos and even full length movies which run perfectly on an Anyka player (which i own) are a pain to watch in a Sunplus.