One of the most confusing things about image metadata is that it consists of 4 parts: embedded (internal, within the actual file coding) metadata, structural/administrative metadata (often external but things like copyright could be embedded; also includes file type) and descriptive metadata (generally external) which we add, things like description (duh, right?), title, keywords/tags, who is in the photo, etc. To add another level of complexity, then there is the website metadata about images, things like alt tag, description, title tag, that sort of thing... (for a quick list of things to include, 10 things you can do to optimize image search -- note> these are all website metadata relating to images, not actually image metadata -- but it is all important metadata)
Website metadata is what is traditionally indexed in search engines, but as some sites (like Flickr) harvest the embedded metadata from an image + allow us to create even more metadata, all of that metadata is made available to search engines via Flickr.
A bit more about embedded metadata
- is created in the process of creating the image. Many phones will embed location and date information - so that when you upload it to flickr or some other website, the site will automatically know where the image was taken and when. It might even know what kind of camera took the photo (it's not really magic, it's metadata). This data is called EXIF (found in JPGs but some other file formats, also). Sort of like a date stamp but inside your image vs. on your image. This metadata can be edited with editors, but often is left as it is.
- is readable by some websites, Flickr and Picasa, among others.
- Some external metadata is also created during the image creation process, e.g., when you take a photo or scan an image, it saves it with a file name. That file name, the size, and file type are external types of metadata but it is more easily accessible to the average user - we change the file names all of the time - though we can't change the file type without using a converter, because the software wouldn't be able to "understand" it. (Just a little tip> if you have a corrupted SD card, chances are you changed metadata to a JPG but not to its matching RAW file, or in some way made the image unreadable via the metadata ... if you are shooting in RAW + JPG and delete a JPG on a SD Card, you must delete the RAW file, too - your camera will usually do this for you; however, not all software will)