If you want to copy an image from one partition to another of identical size, you can do this:
dd if=/dev/hda1 of=/dev/hdb2
is the partition to copy from and hdb2
is the partition to copy to. You can also create an image file:
cat /dev/hda1 | bzip2 > file.img
and the load it back:
cat file.img | bunzip2 > /dev/hdb1
Don't write to the wrong partition or you won't be able to recover. You should also unmount the partition before you copy it; this usually means using a rescue disk.
You can copy an image onto a partition that's too large, but you will then lose the extra space (unless you resize the reiserfs image). If you copy it to a partition that's too small, you'll probably break something.
There are tools out there to do this more efficiently, but this method is simple, needs no extra tools, and it does work.