A little googling led me to this thread on MacRumors.com
It appears that if the ISO is a single session CD image you can just knock the first 600 bytes off with a terminal command and it'll be readable as an ISO (change file type to .iso after performing the command).
I'd recommend making a backup of the .nrg file before trying this though!
$ dd if=image.nrg of=cdrom.iso bs=512 skip=600
(↑ credit to balamw on macrumors forums)
Also, If you're just looking to extract pictures from the disk, File Juicer may also be worth a look!