I have try to extract the Image for WL-500g Premium:

# wlextract WL500gp_1.9.6.8_TW.trx
# dd if=WL500gp_1.9.6.8_TW.trx of=kernel.gz skip=28 bs=1 count=765205

the dd is needed because the wlextrace give you a too short file (1 Byte to small)

# mkdir root_fs
# uncramfs root_fs rootfs


and create a new cramfs Image:

# mkcramfs root_fs root_fs.cramfs
# cat root_fs.cramfs trailer > root_fs-and-trailer

ok that works fine but when i try to create the trx Image i get this warning:

# trx -o my_firmware_wl500gp.trx kernel.gz root_fs-and-trailer
warning: increasing offset 765233 to 765236

and the extract of this new firmware gives a kernel image with 765208 bytes instead of 765204 or 765205

# wlextract my_firmware_wl500gp.trx
# ls -l kernel.gz
-rw-r--r-- 1 root root 765208 May 29 17:59 kernel.gz


Have somebody try this with success???

Tomek