How to find which images belong to which /dev/loop?

losetup (the command normally used to set them up) will tell you:

$ /sbin/losetup --list
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE
/dev/loop0         0      0         0  0 /var/tmp/jigdo/debian-7.6.0-amd64-CD-1.iso

Note that with older versions you may hat to use use -a instead of --list, and this outputs in a different and now deprecated format.

The information comes from /sys:

$ cat /sys/class/block/loop0/loop/backing_file 
/var/tmp/jigdo/debian-7.6.0-amd64-CD-1.iso

Another, possibly more portable, option is to get it from udisks:

$ udisksctl info -b /dev/loop0
/org/freedesktop/UDisks2/block_devices/loop0:
⋮
  org.freedesktop.UDisks2.Loop:
    Autoclear:          false
    BackingFile:        /var/tmp/jigdo/debian-7.6.0-amd64-CD-1.iso
    SetupByUID:         1000
⋮

losetup will also happily remove them for you, using the -d option. That just requires the loop device as a parameter; it doesn't care about the backing file/device.