Does Google Picasa store metadata in the picture files?
Does Google Picasa store metadata in the picture files themselves? If so, what metadata is actually stored inside the files? (as opposed to Picasa's internal database)
Picasa writes tags and captions in the IPTC block (that is in the image file) if the file format supports it. http://picasa.google.com/support/bin/answer.py?hl=en&answer=15055
Since version 3 or so there is also a function to display an iptc tag as album.
(I used that myself and verified that it is actually using iptc via irfanview)
I just tried this with Picasa 3.5.1, and the results were disturbing. In Picasa I changed the caption of a JPEG image from a Pentax K20D camera, then took a look at the file with a metadata utility to see what had been changed. Here is what I found:
The caption was written to the IPTC Caption-Abstract.
All maker note information was COMPLETELY DELETED! (So all information about camera-specific settings is lost.)
The byte order of the EXIF was changed from big-endian to little-endian! (Which goes against the current MWG Recommendation.)
The EXIF software tag was changed! (From "K20D Ver 1.00" to "Picasa 3.0".)
An EXIF ImageUniqueID tag was added.
So beware if you use Picasa to edit metadata.
The answers here that say that Picasa only stores metadata in its internal database and never in the image files are wrong.
Picasa stores some metadata in both its own internal database, and in the image files themselves. As others have pointed out, its handling of image metadata leaves much to be desired.
For example, Picasa 3.5 only supports IPTC metadata to the legacy IPTC-IIM standard. That standard was frozen in 1997. Picasa 3.5 does not yet support the preferred IPTC Core metadata standard, based on XMP. The IPTC Core standard was introduced in 2004. Five years on, and Picasa still hasn't caught up to the fact.
Picasa stores its Caption and tags in the IPTC metadata of the jpg, and the GPS location information in the EXIF metadata.
Try using Irfanview to look at the properties of any jpg that you have tagged and located.