Mounting images that contain only one partition is rather easy. But how to mount image with multiple partitions? kpartx is the solution!

You can list all partitions within the image with

# fdisk -l /vm/dhcpd.img 
Disk /vm/dhcpd.img: 0 MB, 0 bytes
255 heads, 63 sectors/track, 0 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

        Device Boot      Start         End      Blocks   Id  System
/vm/dhcpd.img1   *           1         127     1020096   83  Linux

kpartx can be used to create maps from the block devices

kpartx -a /path/to/xen/image.img

And those maps can be mounted as usual

mount -o rw /dev/mapper/loop0p1 /mnt/