I simply want to embed metadata like a date/time, a title and some tags in the image before I upload the scanned photograph to Flickr. However iPhoto or Lightroom as you can see from my videos seem unfit for the task. Am I missing a setting? Or can someone suggest a better tool?
I cannot speak of iPhoto, because I have not used it much, but Lightroom (LR) does not require you to store multiple versions of images: in fact, one of the key principles in LR is that it does not touch the original image, but stores all changes/updates to an image in its own database. When loading an image another time, it then loads the original image and re-applies the changes/updates before presenting the image on-screen, or export it in one form or another. LR allows you to edit metadata and then export images including the metadata to disk or even directly to Flickr.
The LR workflow does not help you though, if you wish to using VCS's like git.
There are a number of tools however that allow you to edit for example EXIF information in JPG's (assuming you are talking about JPG's). Here is a page dedicated to this: http://www.labnol.org/software/exif-data-editors/14210/. I am not sure if there are any tools that allow adding custom tags to images.
Hope this helps.
I use Adobe Bridge to append metadata to all images. You can create templates and save them. Then you can select as many photos you like and click on the menu "Tools/Append Metadata/[Your previously saved template name]". Then you're all set. It's pretty easy.
You can also edit photos one by one in case you need to fill in some specific information.
Aperture can write IPTC metadata back into the originals. AFAIK, it doesn't modify other metadata, with few exceptions (e.g. you can change the timestamps of photos, very useful if you forgot to update the time in your camera after traveling to a different timezone.)
Like Lightroom, it normally leaves the original intact, and stores the edits in its database. You can change the parameters of each editing at any time, and turn it on and off via a checkbox.
You can create multiple "working copies" of a photo. This doesn't duplicate the original, just the set of edits in the database. This gives you a way to have multiple versions of an image.
So although these features are quite different to a VCS, it gives you similar capabilities, IMHO better suited for a photography workflow.