To search for a specific file or for specific tags, press /, Ctrl+F or F3:
You can choose to search in the file name of the image, for specific tags, or both. You can search for multiple terms, and exclude results by adding "-" to the front of them. Terms are separated by spaces:
That search would find all images with the "wallpaper" tag that don't have the "hl2" tag. Your current search term is shown beside your view order, on the info bar to the right:
To clear this search term and show all images again, press Backspace.