Wikipedia ISO image
Mount ISO image file on FreeBSD

Example
[root@mars mnt]#mdconfig -a -t vnode -f /var/cache1/openSUSE-11.2-DVD-i586.iso -u 1
[root@mars mnt]#mount -t cd9660 /dev/md1 /mnt
[root@mars mnt]#df
Filesystem     Size    Used   Avail Capacity  Mounted on
/dev/da0s1a    1.9G    291M    1.5G    16%    /
devfs          1.0K    1.0K      0B   100%    /dev
/dev/da1s1e     19G     14G    3.3G    81%    /home
/dev/da0s1d    2.8G    522M    2.1G    20%    /tmp
/dev/da0s1e     19G    9.3G    8.1G    54%    /usr
/dev/da1s1d    4.7G    1.2G    3.2G    27%    /var
devfs          1.0K    1.0K      0B   100%    /var/named/dev
/dev/da0s1f     29G    4.2G     23G    16%    /var/cache1
/dev/da1s1f     29G    8.1M     27G     0%    /var/cache2
/dev/md1       4.2G    4.2G      0B   100%    /mnt