Did you install someone else's configuration file that they had shared? If so, please let us know which one so we can inform the person who shared it that they're causing this problem (it's come up at least once before, and Opus doesn't point to ACDSee by default).
HOW TO: Use the Opus image viewer inside or outside of Opus in the FAQs has the steps that should usually work to fix things.
(But if another config has been imported, it can depend on what the config has done. You may also need to look in Settings > File Types at either the JPG type or the special Recognized Images type at the top.)