The different image formats are just various ways of representing the sequence of bits that make up a disc. As such, images don't have a file system because they're not concerned about files and directories, all they're concerned about is lots of 0s and 1s. Whatever image format you use, you'll get the exact same output once it's burnt/mounted/used in some other way.
As for what the filesystem is - (nearly)
all CDs use the ISO 9660 filesystem, with or without various extensions (Rock Ridge - allows long filenames and symlinks; Joliet - allows Unicode filenames). It is technically possibly to put any filesystem you like (e.g. ext2) onto a CD, but the resulting CD wouldn't be readable by any operating systems other than Linux, and I can't think of any reason you'd want to do this asides from curiousity. It's a safe bet that any CD you get hold of that you didn't write yourself has the ISO 9660 filesystem. If you really want to know what extensions are used, it may be stored in the CD-info (I don't know offhand, and can't check at work) which you can get at with the following code (thanks to TLDP
for i in 32768,7 32776,32 32808,32 32958,128 33086,128 33214,128 \
33342,128 33470,32 33581,16 33598,16 33615,16 33632,16
set -- $i
echo "*`dd if=$RD bs=1 skip=$OFFSET count=$LENGTH 2> /dev/null`#"