The search bar above the site tree in the content editor can be used for quick searching if you know part of the file name. This even works for partial matches. For example, searching for “ann parker” will return “Priceless-Wishes-Granted-Wishes-Ann-Parker-1-600px” in the search results. Make sure you have the Media Library selected before running your search. This tells Sitecore to search all child items of the Media Library.
For advanced searching, use the search feature available in the main content area after selecting the Media Library. You can use this to search by tag.
To define custom tags that can be assigned to items, create a new tag underneath /sitecore/system/Settings/Buckets/TagRepository.
To view the tag field on an item (so you can assign tags to that item), make sure you enable “Standard Fields” in the content editor view settings.