If infectus can't flash partial files (I don't know, I don't have one and don't want one), please take your backup, and overwrite the begin of it with a hackimage.
For example (using 'dd' in either windows/cygwin or linux)
cp backup.bin newimage.bin
dd if=hackimage.bin of=newimage.bin conv=notrunc
Please do NOT use a stock 16MB image, as it will not preserve your config data (but rather use the config data of the donor xbox, which might cause trouble in the future).
Normally, you don't need to rebuild the hackimage (unless you want to, of course). But please use the config block of *your* box.