We just published a new version of
Picasa Mobile, 2.7.7, which includes a "Rescan" feature for the local media.
When used, the "rescan" feature will force the Android OS to create new thumbnails for the selected photos.
This was needed because, in certain situations, some thumbnails generated by Android are getting out of sync with their corresponding photos.
A more technical explanation: if the thumbnail information is not properly deleted when deleting a photo, Android OS reuses the thumbnail IDs (instead of generating new ones). Then, when taking a new photo, Android "thinks" that the thumbnail already exists for that photo and it inadvertently reuses the thumbnail of a deleted photo.
Many thanks to Corin for reporting the issue.