Comment 4 for bug 1382329

MAAS 1.7 changed the way in which it imports images. They are now imported into the region controller's database and then synced across all the clusters later.

The API call above that Ian mentions queries what's on the clusters, not what is in the region. Therefore it's possible that the clusters didn't finish syncing, or the region didn't finish importing yet. 1.7 requires users to re-import images even though they may have had them already imported in a previous release before upgrading.