In general, you usually can’t directly modify an image that’s already uploaded on a hosting service unless the service provides its own edit/replace workflow. The common solution is to edit the local original, add the watermark in software such as GIMP, then re-upload or replace the online copy.

For Picasa specifically, one community answer notes that if your local album is still linked/synced to the online album, you may be able to change the watermark settings locally and force Picasa to overwrite the online versions by making a temporary rotation, syncing, then rotating back. That depends on the old Picasa sync workflow.

As for EXIF/metadata, preservation varies by service and by whether the image is replaced or re-exported, so don’t assume it will always be retained. If metadata matters, keep your original files and verify after upload.

So: for most services, no simple server-side watermarking is available after upload; plan on editing originals locally and replacing the online images where possible.