We often use JFFS2 binary images for embedded system, generating them with mkfs.jffs2.
In some instances, it could also be good to mount a jffs2 image into your PC to modify it directly.
To do that, type the following commands as root in a linux shell:
dd if=image-jffs2 of=/dev/mtd0
mount /dev/mtdblock0 mnt
If your image is larger than the default kernel size (4MB in my case), you can try to adjust the mtdram device size with the total_size parameters. when inserting the mtdram module, e.g.:
modprobe mtdram total_size=8096
where total_size is expressed in KB
Further details can be found @ http://www.handhelds.org/hypermail/familiar/62/6232.html