I am not sure .. we probably need a heap more info..
Disk image is on what device? Is it network or locally connected?
Has it worked in the past and failed now or never worked?
Some of Pondini's help files might give you a lead.
There is a few bits about disk images..
I would suspect things like permissions if this is a new disk image.. how have you set up TM to access it, and does it use a password to get in.
If Lapastenague's suggestion doesn't help, a clue may be lurking in your logs. Use the widget in #A1 of Time Machine - Troubleshooting to display the backup messages from your logs. Locate the backup in question, then copy and post all the messages here.